Consistency about byte vs character in string.texi

Consistency about byte vs character in string.texi

* manual/string.texi (String and Array Utilities):
Distinguish more carefully among bytes, multibyte characters,
and wide characters.  Use "byte" when talking about C 'char',
to distinguish it more clearly from multibyte characters.
Say "wide character" or "multibyte character" instead of
"character", when a wide or multibyte character is intended.
Similarly for "multibyte string" versus "string".
Define these terms more carefully.