Document `_process()` and `_physics_process()` delta behavior at low FPS

doc/classes/MainLoop.xml

@@ -78,6 +78,7 @@
 			<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].
 			</description>
 		</method>
 		<method name="_process" qualifiers="virtual">
@@ -86,6 +87,7 @@
 			<description>
 				Called each process (idle) frame with the time since the last process frame as argument (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].
 			</description>
 		</method>
 	</methods>

doc/classes/Node.xml

@@ -71,10 +71,11 @@
 			<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] variable should be constant. [param delta] is in seconds.
+				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].
 				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].
 			</description>
 		</method>
 		<method name="_process" qualifiers="virtual">
@@ -85,6 +86,7 @@
 				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].
 				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].
 			</description>
 		</method>
 		<method name="_ready" qualifiers="virtual">
@@ -490,12 +492,14 @@
 			<return type="float" />
 			<description>
 				Returns the time elapsed (in seconds) since the last physics callback. This value is identical to [method _physics_process]'s [code]delta[/code] parameter, and is often consistent at run-time, unless [member Engine.physics_ticks_per_second] is changed. See also [constant NOTIFICATION_PHYSICS_PROCESS].
+				[b]Note:[/b] The returned value 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 [code]delta[/code] for time measurements in real-world seconds. Use the [Time] singleton's methods for this purpose instead, such as [method Time.get_ticks_usec].
 			</description>
 		</method>
 		<method name="get_process_delta_time" qualifiers="const">
 			<return type="float" />
 			<description>
 				Returns the time elapsed (in seconds) since the last process callback. This value is identical to [method _process]'s [code]delta[/code] parameter, and may vary from frame to frame. See also [constant NOTIFICATION_PROCESS].
+				[b]Note:[/b] The returned value 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 [code]delta[/code] for time measurements in real-world seconds. Use the [Time] singleton's methods for this purpose instead, such as [method Time.get_ticks_usec].
 			</description>
 		</method>
 		<method name="get_scene_instance_load_placeholder" qualifiers="const">