doc, frv: Clean up FRV option documentation [PR122243]

doc, frv: Clean up FRV option documentation [PR122243]

frv.opt has a few options that have never been documented in the
manual.  In the initial commit of the FRV port (prior to the adoption
of .opt files) they were marked as "Internal debug switch" so I have
explicitly made them "Undocumented", consistently with other options
similarly marked in the original port.

The documentation changes all straightforward here, to bring this
section into conformance with conventions being applied through this
chapter of the manual.

gcc/ChangeLog
	PR other/122243
	* config/frv/frv.opt (mbranch-cost=): Mark as Undocumented.
	(mcond-exec-insns=): Likewise.
	(mcond-exec-tempss=): Likewise.
	* doc/invoke.texi (Option Summary) <FRV Options>: Remove duplicate
	positive/negative forms from the list.
	(FRV Options): Combine documentation of positive/negative forms
	where they were listed separately.  Add @opindex entries for
	negative forms.