Rollup merge of rust-lang#147933 - thaliaarchi:consistent-osstring, r=tgross35

os_str: Make platform docs more consistent

- Port `Buf::as_slice`/`as_mut_slice` wording from wtf8 to bytes
- Make `Buf::extend_from_slice_unchecked` docs more platform-independent
- wtf8 `Buf` was missing `#[repr(transparent)]`

Author: matthiaskrgr