Document GPU particles visibility rect/aabb behavior

(cherry picked from commit b4675973fd7287c7540a9393649826bf604dbb65)

doc/classes/Particles.xml

@@ -106,7 +106,8 @@
 			Speed scaling ratio. A value of [code]0[/code] can be used to pause the particles.
 		</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 area of the world part of 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.
+			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] If the [ParticlesMaterial] in use is configured to cast shadows, you may want to enlarge this AABB to ensure the shadow is updated when particles are off-screen.
 		</member>
 	</members>

doc/classes/Particles2D.xml

@@ -78,7 +78,8 @@
 			Particle texture. If [code]null[/code], particles will be squares.
 		</member>
 		<member name="visibility_rect" type="Rect2" setter="set_visibility_rect" getter="get_visibility_rect" default="Rect2( -100, -100, 200, 200 )">
-			Editor visibility helper.
+			The [Rect2] that determines the node's region which needs to be visible on screen for the particle system to be active.
+			Grow the rect if particles suddenly appear/disappear when the node enters/exits the screen. The [Rect2] can be grown via code or with the [b]Particles → Generate Visibility Rect[/b] editor tool.
 		</member>
 	</members>
 	<constants>