Incorrect deprecation notice on some AsciiExt methods

[dtolnay]

For example AsciiExt::is_ascii_hexdigit says:

This method will be deprecated in favor of the identically-named inherent methods on u8, char, [u8] and str.

But we decided not to carry is_ascii_hexdigit to methods on [u8] and str as discussed in #39658 (comment).

The deprecated methods that are not provided for [u8] and str should instead say something like:

This method will be deprecated in favor of the identically-named inherent methods on u8 and char. For [u8] use .iter().all(u8::is_ascii_hexdigit). For str use .bytes().all(|b| b.is_ascii_hexdigit()).

[zajlerke]

I would like to give this a shot if you don't mind.