options: no longer document warnings when diagnosing an invalid -W

options: no longer document warnings when diagnosing an invalid -W

The argmatch functions accept prefixes of the alternatives (like
getopt does for long options).  Bison uses this to document the
warning categories.  This is troublesome: it duplicates the --help
documentation, it is not gettextized, it is displayed with ugly quotes
(because argmatch uses it to display the list of possible answers),
and it prevents straighforward uses of the tables of valid warning
categories (for instance so that warning diagnostics end with the name
of the warning).

The "hidden" option --trace uses the same trick, but it does not need
to be translated, nor to be described in --help.

* src/getargs.c (warnings_args): Remove pseudo documentation.
Comment changes.