Update _physics_process and _process docs to reflect implementation.

doc/classes/MainLoop.xml

@@ -76,18 +76,21 @@
 			<return type="bool" />
 			<param index="0" name="delta" type="float" />
 			<description>
-				Called each physics frame with the time since the last physics frame as argument ([param delta], in seconds). Equivalent to [method Node._physics_process].
-				If implemented, the method must return a boolean value. [code]true[/code] ends the main loop, while [code]false[/code] lets it proceed to the next frame.
-				[b]Note:[/b] [param delta] will be larger than expected if running at a framerate lower than [member Engine.physics_ticks_per_second] / [member Engine.max_physics_steps_per_frame] FPS. This is done to avoid "spiral of death" scenarios where performance would plummet due to an ever-increasing number of physics steps per frame. This behavior affects both [method _process] and [method _physics_process]. As a result, avoid using [param delta] for time measurements in real-world seconds. Use the [Time] singleton's methods for this purpose instead, such as [method Time.get_ticks_usec].
+				Called each physics tick. [param delta] is the logical time between physics ticks in seconds and is equal to [member Engine.time_scale] / [member Engine.physics_ticks_per_second]. Equivalent to [method Node._physics_process].
+				If implemented, the method must return a boolean value. [code]true[/code] ends the main loop, while [code]false[/code] lets it proceed to the next step.
+				[b]Note:[/b] [method _physics_process] may be called up to [member Engine.max_physics_steps_per_frame] times per (idle) frame. This step limit may be reached when the engine is suffering performance issues.
+				[b]Note:[/b] Accumulated [param delta] may diverge from real world seconds.
 			</description>
 		</method>
 		<method name="_process" qualifiers="virtual">
 			<return type="bool" />
 			<param index="0" name="delta" type="float" />
 			<description>
-				Called each process (idle) frame with the time since the last process frame as argument (in seconds). Equivalent to [method Node._process].
+				Called on each idle frame, prior to rendering, and after physics ticks have been processed. [param delta] is the time between frames in seconds. Equivalent to [method Node._process].
 				If implemented, the method must return a boolean value. [code]true[/code] ends the main loop, while [code]false[/code] lets it proceed to the next frame.
-				[b]Note:[/b] [param delta] will be larger than expected if running at a framerate lower than [member Engine.physics_ticks_per_second] / [member Engine.max_physics_steps_per_frame] FPS. This is done to avoid "spiral of death" scenarios where performance would plummet due to an ever-increasing number of physics steps per frame. This behavior affects both [method _process] and [method _physics_process]. As a result, avoid using [param delta] for time measurements in real-world seconds. Use the [Time] singleton's methods for this purpose instead, such as [method Time.get_ticks_usec].
+				[b]Note:[/b] When the engine is struggling and the frame rate is lowered, [param delta] will increase. When [param delta] is increased, it's capped at a maximum of [member Engine.time_scale] * [member Engine.max_physics_steps_per_frame] / [member Engine.physics_ticks_per_second]. As a result, accumulated [param delta] may not represent real world time.
+				[b]Note:[/b] When [code]--fixed-fps[/code] is enabled or the engine is running in Movie Maker mode (see [MovieWriter]), process [param delta] will always be the same for every frame, regardless of how much time the frame took to render.
+				[b]Note:[/b] Frame delta may be post-processed by [member OS.delta_smoothing] if this is enabled for the project.
 			</description>
 		</method>
 	</methods>

doc/classes/Node.xml

@@ -84,24 +84,26 @@
 			<return type="void" />
 			<param index="0" name="delta" type="float" />
 			<description>
-				Called during the physics processing step of the main loop. Physics processing means that the frame rate is synced to the physics, i.e. the [param delta] parameter will [i]generally[/i] be constant (see exceptions below). [param delta] is in seconds.
-				It is only called if physics processing is enabled, which is done automatically if this method is overridden, and can be toggled with [method set_physics_process].
+				Called once on each physics tick, and allows Nodes to synchronize their logic with physics ticks. [param delta] is the logical time between physics ticks in seconds and is equal to [member Engine.time_scale] / [member Engine.physics_ticks_per_second].
+				It is only called if physics processing is enabled for this Node, which is done automatically if this method is overridden, and can be toggled with [method set_physics_process].
 				Processing happens in order of [member process_physics_priority], lower priority values are called first. Nodes with the same priority are processed in tree order, or top to bottom as seen in the editor (also known as pre-order traversal).
 				Corresponds to the [constant NOTIFICATION_PHYSICS_PROCESS] notification in [method Object._notification].
 				[b]Note:[/b] This method is only called if the node is present in the scene tree (i.e. if it's not an orphan).
-				[b]Note:[/b] [param delta] will be larger than expected if running at a framerate lower than [member Engine.physics_ticks_per_second] / [member Engine.max_physics_steps_per_frame] FPS. This is done to avoid "spiral of death" scenarios where performance would plummet due to an ever-increasing number of physics steps per frame. This behavior affects both [method _process] and [method _physics_process]. As a result, avoid using [param delta] for time measurements in real-world seconds. Use the [Time] singleton's methods for this purpose instead, such as [method Time.get_ticks_usec].
+				[b]Note:[/b] Accumulated [param delta] may diverge from real world seconds.
 			</description>
 		</method>
 		<method name="_process" qualifiers="virtual">
 			<return type="void" />
 			<param index="0" name="delta" type="float" />
 			<description>
-				Called during the processing step of the main loop. Processing happens at every frame and as fast as possible, so the [param delta] time since the previous frame is not constant. [param delta] is in seconds.
-				It is only called if processing is enabled, which is done automatically if this method is overridden, and can be toggled with [method set_process].
+				Called on each idle frame, prior to rendering, and after physics ticks have been processed. [param delta] is the time between frames in seconds.
+				It is only called if processing is enabled for this Node, which is done automatically if this method is overridden, and can be toggled with [method set_process].
 				Processing happens in order of [member process_priority], lower priority values are called first. Nodes with the same priority are processed in tree order, or top to bottom as seen in the editor (also known as pre-order traversal).
 				Corresponds to the [constant NOTIFICATION_PROCESS] notification in [method Object._notification].
 				[b]Note:[/b] This method is only called if the node is present in the scene tree (i.e. if it's not an orphan).
-				[b]Note:[/b] [param delta] will be larger than expected if running at a framerate lower than [member Engine.physics_ticks_per_second] / [member Engine.max_physics_steps_per_frame] FPS. This is done to avoid "spiral of death" scenarios where performance would plummet due to an ever-increasing number of physics steps per frame. This behavior affects both [method _process] and [method _physics_process]. As a result, avoid using [param delta] for time measurements in real-world seconds. Use the [Time] singleton's methods for this purpose instead, such as [method Time.get_ticks_usec].
+				[b]Note:[/b] When the engine is struggling and the frame rate is lowered, [param delta] will increase. When [param delta] is increased, it's capped at a maximum of [member Engine.time_scale] * [member Engine.max_physics_steps_per_frame] / [member Engine.physics_ticks_per_second]. As a result, accumulated [param delta] may not represent real world time.
+				[b]Note:[/b] When [code]--fixed-fps[/code] is enabled or the engine is running in Movie Maker mode (see [MovieWriter]), process [param delta] will always be the same for every frame, regardless of how much time the frame took to render.
+				[b]Note:[/b] Frame delta may be post-processed by [member OS.delta_smoothing] if this is enabled for the project.
 			</description>
 		</method>
 		<method name="_ready" qualifiers="virtual">