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.

lisp/emacs-lisp/lisp-mode.el

@@ -1453,7 +1453,7 @@ and initial semicolons."
       ;; are buffer-local, but we avoid changing them so that they can be set
       ;; to make `forward-paragraph' and friends do something the user wants.
       ;;
-      ;; `paragraph-start': The `(' in the character alternative and the
+      ;; `paragraph-start': The `(' in the bracket expression and the
       ;; left-singlequote plus `(' sequence after the \\| alternative prevent
       ;; sexps and backquoted sexps that follow a docstring from being filled
       ;; with the docstring.  This setting has the consequence of inhibiting