Call them “bracket expressions” more consistently

Call them “bracket expressions” more consistently

Emacs comments and doc were inconsistent about the name used for
regexps like [a-z].  Sometimes it called them “character
alternatives”, sometimes “character sets”, sometimes “bracket
expressions”.  Prefer “bracket expressions” as it is less confusing:
POSIX and most other programs’ doc uses “bracket expressions”,
“alternative” is also used in the Emacs documentation to talk about
...\|... in regexps, and “character set” normally has a different
meaning in Emacs.