Document relationship between refresh rate and V-Sync in DisplayServer

[Calinou]

• Add search keyword to make DisplayServer.screen_get_refresh_rate() easier to find.
• Mention formula for optimal FPS capping when VRR and V-Sync are both enabled. This is the same formula as used in Reflex.

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• See V-Sync value godot-proposals#13334 (comment).

[Mickeon]

This mentions "a VRR window", but it isn't readily clear what that is. We know it's "Variable Refresh Rate", but the first mention is in lowercase (which is appropriate).
Nitpicky, too, but "window", especially where this setting is located, can be interpreted as the actual Window class, not a window of time.
Similar case for "the margin". What margin?

Suggested change

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 margin needs to be increased to account for timing inaccuracies and ensure the rendering remains within the VRR window. The optimal formula for this is [code]max_fps = 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]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]. If the refresh rate is high, the FPS limit should be increased in order to ensure frames are rendered in time and to account for timing inaccuracies. The optimal formula for this setting is [code]r - (r * r) / 3600.0[/code], where [code]r[/code] is the monitor's refresh rate.

Or similar. I'm not sure I have the technical knowhow myself to describe it.

I'm also not sure how beneficial this information is exactly, in the project settings. It's not like users can adapt this setting at runtime to match an user's refresh rate. All of this feels more fitting to mention in Engine.max_fps, instead.

[Calinou]

I can't find a quick resource that explains what the VRR window is, other than a reply on a Reddit thread: https://www.reddit.com/r/digitalfoundry/comments/18vdgkz/vrr_window/kfq9jiw/

It's a fairly simple concept at its core (a range between two refresh rates), but there are some subtleties like low frame compensation and different displays having different minimum refresh rates. The lower bound of the VRR window is typically 48 Hz, or 24 Hz when taking LFC into account, but not always.

I guess "VRR range" is less confusing still, so I went ahead and replaced "VRR window" with it. I've also added this information to Engine.max_fps's description.

Note that ideally, the engine should have an option to do this on its own:

• Implement automatic FPS limiting based on highest display refresh rate #66367