Improve documentation related for particle subemitters, collision and attractors

This also describes how to set up particle flipbook animation in
GPUParticles2D's texture property.

doc/classes/BaseMaterial3D.xml

@@ -737,7 +737,7 @@
 			The object's X axis will always face the camera.
 		</constant>
 		<constant name="BILLBOARD_PARTICLES" value="3" enum="BillboardMode">
-			Used for particle systems when assigned to [GPUParticles3D] and [CPUParticles3D] nodes. Enables [code]particles_anim_*[/code] properties.
+			Used for particle systems when assigned to [GPUParticles3D] and [CPUParticles3D] nodes (flipbook animation). Enables [code]particles_anim_*[/code] properties.
 			The [member ParticleProcessMaterial.anim_speed_min] or [member CPUParticles3D.anim_speed_min] should also be set to a value bigger than zero for the animation to play.
 		</constant>
 		<constant name="TEXTURE_CHANNEL_RED" value="0" enum="TextureChannel">

doc/classes/GPUParticles2D.xml

@@ -48,14 +48,16 @@
 	</methods>
 	<members>
 		<member name="amount" type="int" setter="set_amount" getter="get_amount" default="8">
-			Number of particles emitted in one emission cycle.
+			The number of particles to emit in one emission cycle. The effective emission rate is [code](amount * amount_ratio) / lifetime[/code] particles per second. Higher values will increase GPU requirements, even if not all particles are visible at a given time or if [member amount_ratio] is decreased.
+			[b]Note:[/b] Changing this value will cause the particle system to restart. To avoid this, change [member amount_ratio] instead.
 		</member>
 		<member name="amount_ratio" type="float" setter="set_amount_ratio" getter="get_amount_ratio" default="1.0">
 			The ratio of particles that should actually be emitted. If set to a value lower than [code]1.0[/code], this will set the amount of emitted particles throughout the lifetime to [code]amount * amount_ratio[/code]. Unlike changing [member amount], changing [member amount_ratio] while emitting does not affect already-emitted particles and doesn't cause the particle system to restart. [member amount_ratio] can be used to create effects that make the number of emitted particles vary over time.
-			[b]Note:[/b] Reducing the [member amount_ratio] has no performance benefit, since resources need to be allocated and processed for the total [member amount] of particles regardless of the [member amount_ratio].
+			[b]Note:[/b] Reducing the [member amount_ratio] has no performance benefit, since resources need to be allocated and processed for the total [member amount] of particles regardless of the [member amount_ratio]. If you don't intend to change the number of particles emitted while the particles are emitting, make sure [member amount_ratio] is set to [code]1[/code] and change [member amount] to your liking instead.
 		</member>
 		<member name="collision_base_size" type="float" setter="set_collision_base_size" getter="get_collision_base_size" default="1.0">
-			Multiplier for particle's collision radius. [code]1.0[/code] corresponds to the size of the sprite.
+			Multiplier for particle's collision radius. [code]1.0[/code] corresponds to the size of the sprite. If particles appear to sink into the ground when colliding, increase this value. If particles appear to float when colliding, decrease this value. Only effective if [member ParticleProcessMaterial.collision_mode] is [constant ParticleProcessMaterial.COLLISION_RIGID] or [constant ParticleProcessMaterial.COLLISION_HIDE_ON_CONTACT].
+			[b]Note:[/b] Particles always have a spherical collision shape.
 		</member>
 		<member name="draw_order" type="int" setter="set_draw_order" getter="get_draw_order" enum="GPUParticles2D.DrawOrder" default="1">
 			Particle draw order. Uses [enum DrawOrder] values.
@@ -80,7 +82,7 @@
 			Enables particle interpolation, which makes the particle movement smoother when their [member fixed_fps] is lower than the screen refresh rate.
 		</member>
 		<member name="lifetime" type="float" setter="set_lifetime" getter="get_lifetime" default="1.0">
-			Amount of time each particle will exist.
+			The amount of time each particle will exist (in seconds). The effective emission rate is [code](amount * amount_ratio) / lifetime[/code] particles per second.
 		</member>
 		<member name="local_coords" type="bool" setter="set_use_local_coordinates" getter="get_use_local_coordinates" default="false">
 			If [code]true[/code], particles use the parent node's coordinate space (known as local coordinates). This will cause particles to move and rotate along the [GPUParticles2D] node (and its parents) when it is moved or rotated. If [code]false[/code], particles use global coordinates; they will not move or rotate along the [GPUParticles2D] node (and its parents) when it is moved or rotated.
@@ -101,10 +103,12 @@
 			Particle system's running speed scaling ratio. A value of [code]0[/code] can be used to pause the particles.
 		</member>
 		<member name="sub_emitter" type="NodePath" setter="set_sub_emitter" getter="get_sub_emitter" default="NodePath(&quot;&quot;)">
-			The [NodePath] to the [GPUParticles2D] used for sub-emissions.
+			Path to another [GPUParticles2D] node that will be used as a subemitter (see [member ParticleProcessMaterial.sub_emitter_mode]). Subemitters can be used to achieve effects such as fireworks, sparks on collision, bubbles popping into water drops, and more.
+			[b]Note:[/b] When [member sub_emitter] is set, the target [GPUParticles2D] node will no longer emit particles on its own.
 		</member>
 		<member name="texture" type="Texture2D" setter="set_texture" getter="get_texture">
-			Particle texture. If [code]null[/code], particles will be squares.
+			Particle texture. If [code]null[/code], particles will be squares with a size of 1×1 pixels.
+			[b]Note:[/b] To use a flipbook texture, assign a new [CanvasItemMaterial] to the [GPUParticles2D]'s [member CanvasItem.material] property, then enable [member CanvasItemMaterial.particles_animation] and set [member CanvasItemMaterial.particles_anim_h_frames], [member CanvasItemMaterial.particles_anim_v_frames], and [member CanvasItemMaterial.particles_anim_loop] to match the flipbook texture.
 		</member>
 		<member name="trail_enabled" type="bool" setter="set_trail_enabled" getter="is_trail_enabled" default="false">
 			If [code]true[/code], enables particle trails using a mesh skinning system.

doc/classes/GPUParticles3D.xml

@@ -61,13 +61,16 @@
 	</methods>
 	<members>
 		<member name="amount" type="int" setter="set_amount" getter="get_amount" default="8">
-			Number of particles to emit.
+			The number of particles to emit in one emission cycle. The effective emission rate is [code](amount * amount_ratio) / lifetime[/code] particles per second. Higher values will increase GPU requirements, even if not all particles are visible at a given time or if [member amount_ratio] is decreased.
+			[b]Note:[/b] Changing this value will cause the particle system to restart. To avoid this, change [member amount_ratio] instead.
 		</member>
 		<member name="amount_ratio" type="float" setter="set_amount_ratio" getter="get_amount_ratio" default="1.0">
 			The ratio of particles that should actually be emitted. If set to a value lower than [code]1.0[/code], this will set the amount of emitted particles throughout the lifetime to [code]amount * amount_ratio[/code]. Unlike changing [member amount], changing [member amount_ratio] while emitting does not affect already-emitted particles and doesn't cause the particle system to restart. [member amount_ratio] can be used to create effects that make the number of emitted particles vary over time.
-			[b]Note:[/b] Reducing the [member amount_ratio] has no performance benefit, since resources need to be allocated and processed for the total [member amount] of particles regardless of the [member amount_ratio].
+			[b]Note:[/b] Reducing the [member amount_ratio] has no performance benefit, since resources need to be allocated and processed for the total [member amount] of particles regardless of the [member amount_ratio]. If you don't intend to change the number of particles emitted while the particles are emitting, make sure [member amount_ratio] is set to [code]1[/code] and change [member amount] to your liking instead.
 		</member>
 		<member name="collision_base_size" type="float" setter="set_collision_base_size" getter="get_collision_base_size" default="0.01">
+			The base diameter for particle collision in meters. If particles appear to sink into the ground when colliding, increase this value. If particles appear to float when colliding, decrease this value. Only effective if [member ParticleProcessMaterial.collision_mode] is [constant ParticleProcessMaterial.COLLISION_RIGID] or [constant ParticleProcessMaterial.COLLISION_HIDE_ON_CONTACT].
+			[b]Note:[/b] Particles always have a spherical collision shape.
 		</member>
 		<member name="draw_order" type="int" setter="set_draw_order" getter="get_draw_order" enum="GPUParticles3D.DrawOrder" default="0">
 			Particle draw order. Uses [enum DrawOrder] values.
@@ -110,7 +113,7 @@
 			Enables particle interpolation, which makes the particle movement smoother when their [member fixed_fps] is lower than the screen refresh rate.
 		</member>
 		<member name="lifetime" type="float" setter="set_lifetime" getter="get_lifetime" default="1.0">
-			Amount of time each particle will exist.
+			The amount of time each particle will exist (in seconds). The effective emission rate is [code](amount * amount_ratio) / lifetime[/code] particles per second.
 		</member>
 		<member name="local_coords" type="bool" setter="set_use_local_coordinates" getter="get_use_local_coordinates" default="false">
 			If [code]true[/code], particles use the parent node's coordinate space (known as local coordinates). This will cause particles to move and rotate along the [GPUParticles3D] node (and its parents) when it is moved or rotated. If [code]false[/code], particles use global coordinates; they will not move or rotate along the [GPUParticles3D] node (and its parents) when it is moved or rotated.
@@ -131,6 +134,8 @@
 			Speed scaling ratio. A value of [code]0[/code] can be used to pause the particles.
 		</member>
 		<member name="sub_emitter" type="NodePath" setter="set_sub_emitter" getter="get_sub_emitter" default="NodePath(&quot;&quot;)">
+			Path to another [GPUParticles3D] node that will be used as a subemitter (see [member ParticleProcessMaterial.sub_emitter_mode]). Subemitters can be used to achieve effects such as fireworks, sparks on collision, bubbles popping into water drops, and more.
+			[b]Note:[/b] When [member sub_emitter] is set, the target [GPUParticles3D] node will no longer emit particles on its own.
 		</member>
 		<member name="trail_enabled" type="bool" setter="set_trail_enabled" getter="is_trail_enabled" default="false">
 			If [code]true[/code], enables particle trails using a mesh skinning system. Designed to work with [RibbonTrailMesh] and [TubeTrailMesh].
@@ -143,8 +148,9 @@
 		<member name="transform_align" type="int" setter="set_transform_align" getter="get_transform_align" enum="GPUParticles3D.TransformAlign" default="0">
 		</member>
 		<member name="visibility_aabb" type="AABB" setter="set_visibility_aabb" getter="get_visibility_aabb" default="AABB(-4, -4, -4, 8, 8, 8)">
-			The [AABB] that determines the node's region which needs to be visible on screen for the particle system to be active.
+			The [AABB] that determines the node's region which needs to be visible on screen for the particle system to be active. [member GeometryInstance3D.extra_cull_margin] is added on each of the AABB's axes. Particle collisions and attraction will only occur within this area.
 			Grow the box if particles suddenly appear/disappear when the node enters/exits the screen. The [AABB] can be grown via code or with the [b]Particles → Generate AABB[/b] editor tool.
+			[b]Note:[/b] [member visibility_aabb] is overridden by [member GeometryInstance3D.custom_aabb] if that property is set to a non-default value.
 		</member>
 	</members>
 	<signals>

doc/classes/ParticleProcessMaterial.xml

@@ -114,7 +114,7 @@
 			Minimum equivalent of [member anim_speed_max].
 		</member>
 		<member name="attractor_interaction_enabled" type="bool" setter="set_attractor_interaction_enabled" getter="is_attractor_interaction_enabled" default="true">
-			True if the interaction with particle attractors is enabled.
+			If [code]true[/code], interaction with particle attractors is enabled. In 3D, attraction only occurs within the area defined by the [GPUParticles3D] node's [member GPUParticles3D.visibility_aabb].
 		</member>
 		<member name="collision_bounce" type="float" setter="set_collision_bounce" getter="get_collision_bounce">
 			The particles' bounciness. Values range from [code]0[/code] (no bounce) to [code]1[/code] (full bounciness). Only effective if [member collision_mode] is [constant COLLISION_RIGID].
@@ -124,11 +124,11 @@
 		</member>
 		<member name="collision_mode" type="int" setter="set_collision_mode" getter="get_collision_mode" enum="ParticleProcessMaterial.CollisionMode" default="0">
 			The particles' collision mode.
-			[b]Note:[/b] 3D Particles can only collide with [GPUParticlesCollision3D] nodes, not [PhysicsBody3D] nodes. To make particles collide with various objects, you can add [GPUParticlesCollision3D] nodes as children of [PhysicsBody3D] nodes.
+			[b]Note:[/b] 3D Particles can only collide with [GPUParticlesCollision3D] nodes, not [PhysicsBody3D] nodes. To make particles collide with various objects, you can add [GPUParticlesCollision3D] nodes as children of [PhysicsBody3D] nodes. In 3D, collisions only occur within the area defined by the [GPUParticles3D] node's [member GPUParticles3D.visibility_aabb].
 			[b]Note:[/b] 2D Particles can only collide with [LightOccluder2D] nodes, not [PhysicsBody2D] nodes.
 		</member>
 		<member name="collision_use_scale" type="bool" setter="set_collision_use_scale" getter="is_collision_using_scale" default="false">
-			Should collision take scale into account.
+			If [code]true[/code], [member GPUParticles3D.collision_base_size] is multiplied by the particle's effective scale (see [member scale_min], [member scale_max], [member scale_curve], and [member scale_over_velocity_curve]).
 		</member>
 		<member name="color" type="Color" setter="set_color" getter="get_color" default="Color(1, 1, 1, 1)">
 			Each particle's initial color. If the [GPUParticles2D]'s [code]texture[/code] is defined, it will be multiplied by this color.
@@ -316,16 +316,22 @@
 			Each particle's initial direction range from [code]+spread[/code] to [code]-spread[/code] degrees.
 		</member>
 		<member name="sub_emitter_amount_at_collision" type="int" setter="set_sub_emitter_amount_at_collision" getter="get_sub_emitter_amount_at_collision">
-			Sub particle amount on collision.
-			Maximum amount set in the sub particles emitter.
+			The amount of particles to spawn from the subemitter node when a collision occurs. When combined with [constant COLLISION_HIDE_ON_CONTACT] on the main particles material, this can be used to achieve effects such as raindrops hitting the ground.
+			[b]Note:[/b] This value shouldn't exceed [member GPUParticles2D.amount] or [member GPUParticles3D.amount] defined on the [i]subemitter node[/i] (not the main node), relative to the subemitter's particle lifetime. If the number of particles is exceeded, no new particles will spawn from the subemitter until enough particles have expired.
 		</member>
 		<member name="sub_emitter_amount_at_end" type="int" setter="set_sub_emitter_amount_at_end" getter="get_sub_emitter_amount_at_end">
+			The amount of particles to spawn from the subemitter node when the particle expires.
+			[b]Note:[/b] This value shouldn't exceed [member GPUParticles2D.amount] or [member GPUParticles3D.amount] defined on the [i]subemitter node[/i] (not the main node), relative to the subemitter's particle lifetime. If the number of particles is exceeded, no new particles will spawn from the subemitter until enough particles have expired.
 		</member>
 		<member name="sub_emitter_frequency" type="float" setter="set_sub_emitter_frequency" getter="get_sub_emitter_frequency">
+			The frequency at which particles should be emitted from the subemitter node. One particle will be spawned every [member sub_emitter_frequency] seconds.
+			[b]Note:[/b] This value shouldn't exceed [member GPUParticles2D.amount] or [member GPUParticles3D.amount] defined on the [i]subemitter node[/i] (not the main node), relative to the subemitter's particle lifetime. If the number of particles is exceeded, no new particles will spawn from the subemitter until enough particles have expired.
 		</member>
 		<member name="sub_emitter_keep_velocity" type="bool" setter="set_sub_emitter_keep_velocity" getter="get_sub_emitter_keep_velocity" default="false">
+			If [code]true[/code], the subemitter inherits the parent particle's velocity when it spawns.
 		</member>
 		<member name="sub_emitter_mode" type="int" setter="set_sub_emitter_mode" getter="get_sub_emitter_mode" enum="ParticleProcessMaterial.SubEmitterMode" default="0">
+			The particle subemitter mode (see [member GPUParticles2D.sub_emitter] and [member GPUParticles3D.sub_emitter]).
 		</member>
 		<member name="tangential_accel_curve" type="Texture2D" setter="set_param_texture" getter="get_param_texture">
 			Each particle's tangential acceleration will vary along this [CurveTexture].