Add docs to `RawHandleWrapper::_window` field

[alice-i-cecile]

It's needed to drop the window at the correct time. See this comment on window wrapper. https://docs.rs/bevy/latest/bevy/window/struct.WindowWrapper.html

Originally posted by @hymm in #15380 (comment)

[alice-i-cecile]

Just copying over the docs from there are fine: it would be good to avoid a repeat of the linked PR.