Document whether ControlFlow::WaitUntil(deadline_in_past) handles or drops event

[fritzrehde]

The docs for https://docs.rs/winit/latest/winit/event_loop/enum.ControlFlow.html#variant.WaitUntil say:

When the current loop iteration finishes, suspend the thread until either another event arrives or the given time is reached.

These docs don't clearly state whether, if we pass an instant to WaitUntil that is in the past, that event gets handled or dropped. Could you make "or the given time is reached" a little more clear. Thanks!