Improve docs for ReceivedCharacter

[rparrett]

How can Bevy's documentation be improved?

Since #10702, ReceivedCharacter now contains SmolStr, which is a bit confusing. Is it a character or multiple characters?

The winit docs for the underlying winit event that populates this have some info about why it is not a char and what else might be in there. (but surely this isn't a str just for stray deadkeys, right? we should look into what locales or settings or whatever would produce multiple chars.)

ReceivedCharacter is also not consistent cross-platform in other important ways. See rust-windowing/winit#3381.

We might also want to point users to an alternative: KeyEvent::logical_key which is more consistent across platforms and is in a unified event stream with other keyboard events, which is important if you are developing a text input.

[james7132]

Closing as the type is to be deprecated and removed: #12639.