Document relationship between refresh rate and V-Sync in DisplayServer

- Mention formula for optimal FPS capping when VRR and V-Sync are both
  enabled. This is the same formula as used in Reflex.

Author: Calinou

doc/classes/DisplayServer.xml

@@ -1774,11 +1774,11 @@
 				[b]Note:[/b] One of the following constants can be used as [param screen]: [constant SCREEN_OF_MAIN_WINDOW], [constant SCREEN_PRIMARY], [constant SCREEN_WITH_MOUSE_FOCUS], or [constant SCREEN_WITH_KEYBOARD_FOCUS].
 			</description>
 		</method>
-		<method name="screen_get_refresh_rate" qualifiers="const">
+		<method name="screen_get_refresh_rate" qualifiers="const" keywords="vsync">
 			<return type="float" />
 			<param index="0" name="screen" type="int" default="-1" />
 			<description>
-				Returns the current refresh rate of the specified screen. Returns [code]-1.0[/code] if [param screen] is invalid or the [DisplayServer] fails to find the refresh rate for the specified screen.
+				Returns the current refresh rate of the specified screen. When V-Sync is enabled, this returns the maximum framerate the project can effectively reach. Returns [code]-1.0[/code] if [param screen] is invalid or the [DisplayServer] fails to find the refresh rate for the specified screen.
 				To fallback to a default refresh rate if the method fails, try:
 				[codeblock]
 				var refresh_rate = DisplayServer.screen_get_refresh_rate()

doc/classes/Engine.xml

@@ -316,8 +316,7 @@
 			The maximum number of frames that can be rendered every second (FPS). A value of [code]0[/code] means the framerate is uncapped.
 			Limiting the FPS can be useful to reduce the host machine's power consumption, which reduces heat, noise emissions, and improves battery life.
 			If [member ProjectSettings.display/window/vsync/vsync_mode] is [b]Enabled[/b] or [b]Adaptive[/b], the setting takes precedence and the max FPS number cannot exceed the monitor's refresh rate.
-			If [member ProjectSettings.display/window/vsync/vsync_mode] is [b]Enabled[/b], on monitors with variable refresh rate enabled (G-Sync/FreeSync), using an FPS limit a few frames lower than the monitor's refresh rate will [url=https://blurbusters.com/howto-low-lag-vsync-on/]reduce input lag while avoiding tearing[/url].
-			See also [member physics_ticks_per_second] and [member ProjectSettings.application/run/max_fps].
+			If [member ProjectSettings.display/window/vsync/vsync_mode] is [b]Enabled[/b], on monitors with variable refresh rate enabled (G-Sync/FreeSync), using an FPS limit a few frames lower than the monitor's refresh rate will [url=https://blurbusters.com/howto-low-lag-vsync-on/]reduce input lag while avoiding tearing[/url]. At higher refresh rates, the difference between the FPS limit and the monitor refresh rate should be increased to ensure frames to account for timing inaccuracies. The optimal formula for this is [code]r - (r * r) / 3600.0[/code], where [code]r[/code] is the monitor's refresh rate.
 			[b]Note:[/b] The actual number of frames per second may still be below this value if the CPU or GPU cannot keep up with the project's logic and rendering.
 			[b]Note:[/b] If [member ProjectSettings.display/window/vsync/vsync_mode] is [b]Disabled[/b], limiting the FPS to a high value that can be consistently reached on the system can reduce input lag compared to an uncapped framerate. Since this works by ensuring the GPU load is lower than 100%, this latency reduction is only effective in GPU-bottlenecked scenarios, not CPU-bottlenecked scenarios.
 		</member>

doc/classes/ProjectSettings.xml

@@ -394,8 +394,8 @@
 		<member name="application/run/max_fps" type="int" setter="" getter="" default="0">
 			Maximum number of frames per second allowed. A value of [code]0[/code] means "no limit". The actual number of frames per second may still be below this value if the CPU or GPU cannot keep up with the project logic and rendering.
 			Limiting the FPS can be useful to reduce system power consumption, which reduces heat and noise emissions (and improves battery life on mobile devices).
-			If [member display/window/vsync/vsync_mode] is set to [code]Enabled[/code] or [code]Adaptive[/code], it takes precedence and the forced FPS number cannot exceed the monitor's refresh rate.
-			If [member display/window/vsync/vsync_mode] is [code]Enabled[/code], on monitors with variable refresh rate enabled (G-Sync/FreeSync), using an FPS limit a few frames lower than the monitor's refresh rate will [url=https://blurbusters.com/howto-low-lag-vsync-on/]reduce input lag while avoiding tearing[/url].
+			If [member display/window/vsync/vsync_mode] is set to [code]Enabled[/code] or [code]Adaptive[/code], it takes precedence and the forced FPS number cannot exceed the monitor's refresh rate. See also [method DisplayServer.screen_get_refresh_rate].
+			If [member display/window/vsync/vsync_mode] is [code]Enabled[/code], on monitors with variable refresh rate enabled (G-Sync/FreeSync), using an FPS limit slightly lower than the monitor's refresh rate will [url=https://blurbusters.com/howto-low-lag-vsync-on/]reduce input lag while avoiding tearing[/url]. At higher refresh rates, the difference between the FPS limit and the monitor refresh rate should be increased to ensure frames to account for timing inaccuracies. The optimal formula for this is [code]r - (r * r) / 3600.0[/code], where [code]r[/code] is the monitor's refresh rate.
 			If [member display/window/vsync/vsync_mode] is [code]Disabled[/code], limiting the FPS to a high value that can be consistently reached on the system can reduce input lag compared to an uncapped framerate. Since this works by ensuring the GPU load is lower than 100%, this latency reduction is only effective in GPU-bottlenecked scenarios, not CPU-bottlenecked scenarios.
 			See also [member physics/common/physics_ticks_per_second].
 			This setting can be overridden using the [code]--max-fps &lt;fps&gt;[/code] command line argument (including with a value of [code]0[/code] for unlimited framerate).