doc: Add "user experience" documentation for options

doc: Add "user experience" documentation for options

After working through PR122243, it seems appropriate to write down
some guidelines for adding/maintaining options and options
documentation so that we can at least point to procedures to keep
things from getting out of sync again.

Thanks to jemarch@gnu.org and dmalcolm@redhat.com for their suggestions
to improve this patch.

gcc/ChangeLog
	* doc/options.texi (Options): Point to the "user experience"
	documentation.
	* doc/ux.texi (Guidelines for Options): Add some.