doc: don't refer to 'char' as characters

This seems to be causing some confusion among users. Rust's char are
not 8bit characters, but 32bit UCS-4 codepoint without surrogates
(Unicode Scalar Values as per Unicode glossary).
Make the doc more explicit about it.

Signed-off-by: Luca Bruno <lucab@debian.org>
doc: uniform std::char doc-strings

Uniform and beautify doc-string for current rustdoc output.

Signed-off-by: Luca Bruno <lucab@debian.org>
doc: add two missing char methods doc-strings

XID_* property are defined in UAX #31, just reference it here.

Signed-off-by: Luca Bruno <lucab@debian.org>