Document buffering behavior for events

[thurn]

The documentation for event::read doesn't discuss buffering behavior: https://docs.rs/crossterm/0.27.0/crossterm/event/fn.read.html, i.e. what happens if multiple events have come in since the last time you checked for events.

Based on briefly looking at the source code I think the behavior is to keep the most recent 32 events and silently drop the rest, returning them in order from oldest to newest. But I could also be reading it completely wrong.