doc: clarify what's counted by wc

doc: clarify what's counted by wc

* src/wc.c (usage): State that only printable characters are considered
when counting words.  This also disambiguates wether we're talking
about bytes or characters in this context.
* doc/coreutils.texi (wc invocation): Likewise.  Also clarify
that --characters counts valid locale aware characters,
and that --lines does not count a trailing "line" unless
it ends with a newline character.
Fixes https://bugs.gnu.org/47702