Mark some undocumented options as such [PR122243]

Mark some undocumented options as such [PR122243]

We have a number of command-line options that are undocumented (either
intentionally or because they are obsolete and retained only for
compatibility), that ought to be marked as "Undocumented".  I've also
added some comments to the .opt files.

gcc/c-family/ChangeLog
	PR other/122243
	* c.opt (fmodule-version-ignore): Mark as "Undocumented".

gcc/ChangeLog
	PR other/122243
	* common.opt (fhelp, fhelp=, ftarget-help, fversion): Mark as
	"Undocumented".
	(fbounds-check): Update comments.
	(flag-graphite, fsel-sched-reschedule-pipelined): Mark as
	"Undocumented".
	(fstack-limit): Add comment.