Improve documentation related to fixed physics processing

(cherry picked from commit b89a37269b)

doc/classes/Engine.xml

@@ -147,10 +147,10 @@
 			If [code]true[/code], it is running inside the editor. Useful for tool scripts.
 		</member>
 		<member name="iterations_per_second" type="int" setter="set_iterations_per_second" getter="get_iterations_per_second" default="60">
-			The number of fixed iterations per second (for fixed process and physics).
+			The number of fixed iterations per second. This controls how often physics simulation and [method Node._physics_process] methods are run. This value should generally always be set to [code]60[/code] or above, as Godot doesn't interpolate the physics step. As a result, values lower than [code]60[/code] will look stuttery. This value can be increased to make input more reactive or work around tunneling issues, but keep in mind doing so will increase CPU usage.
 		</member>
 		<member name="physics_jitter_fix" type="float" setter="set_physics_jitter_fix" getter="get_physics_jitter_fix" default="0.5">
-			Controls how much physic ticks are synchronized  with real time. For 0 or less, the ticks are synchronized. Such values are recommended for network games, where clock synchronization matters. Higher values cause higher deviation of in-game clock and real clock, but allows to smooth out framerate jitters. The default value of 0.5 should be fine for most; values above 2 could cause the game to react to dropped frames with a noticeable delay and are not recommended.
+			Controls how much physics ticks are synchronized with real time. For 0 or less, the ticks are synchronized. Such values are recommended for network games, where clock synchronization matters. Higher values cause higher deviation of in-game clock and real clock, but allows to smooth out framerate jitters. The default value of 0.5 should be fine for most; values above 2 could cause the game to react to dropped frames with a noticeable delay and are not recommended.
 		</member>
 		<member name="target_fps" type="int" setter="set_target_fps" getter="get_target_fps" default="0">
 			The desired frames per second. If the hardware cannot keep up, this setting may not be respected. A value of 0 means no limit.

doc/classes/ProjectSettings.xml

@@ -956,10 +956,12 @@
 			Enables [member Viewport.physics_object_picking] on the root viewport.
 		</member>
 		<member name="physics/common/physics_fps" type="int" setter="" getter="" default="60">
-			Frames per second used in the physics. Physics always needs a fixed amount of frames per second.
+			The number of fixed iterations per second. This controls how often physics simulation and [method Node._physics_process] methods are run.
+			[b]Note:[/b] This property is only read when the project starts. To change the physics FPS at runtime, set [member Engine.iterations_per_second] instead.
 		</member>
 		<member name="physics/common/physics_jitter_fix" type="float" setter="" getter="" default="0.5">
 			Fix to improve physics jitter, specially on monitors where refresh rate is different than the physics FPS.
+			[b]Note:[/b] This property is only read when the project starts. To change the physics FPS at runtime, set [member Engine.physics_jitter_fix] instead.
 		</member>
 		<member name="rendering/environment/default_clear_color" type="Color" setter="" getter="" default="Color( 0.3, 0.3, 0.3, 1 )">
 			Default background clear color. Overridable per [Viewport] using its [Environment]. See [member Environment.background_mode] and [member Environment.background_color] in particular. To change this default color programmatically, use [method VisualServer.set_default_clear_color].