doc: Mark more options as Undocumented/RejectNegative [PR122243]

doc: Mark more options as Undocumented/RejectNegative [PR122243]

In reviewing the autogenerated list of remaining undocumented options
after my first pass through the whole chapter, I found several more
options I'd previously overlooked that should either be marked
Undocumented, or that were missing a RejectNegative attribute where
one was plainly appropriate.

gcc/c-family/ChangeLog
	PR other/122243
	* c.opt (-output-pch): Mark as Undocumented, as it seems to be
	an internal option that has never been documented anyway.
	(Werror-implicit-function-declaration): Mark deprecated option
	that is not currently documented as Undocumented.
	(fconstant-string-class=): Add RejectNegative property.

gcc/ChangeLog
	PR other/122243
	* common.opt (fbounds-check): Mark as Undocumented, expand comments
	to explain why.
	* config/frv/frv.opt (msched-lookahead=): Mark unused option as
	Undocumented.
	* config/m68k/m68k.opt (m68851): Add RejectNegative.
	* config/nvptx/nvptx.opt (minit-regs=): Mark as Undocumented.  It's
	not currently documented and seems to have been introduced as a
	stopgap to experiment with different implementation strategies.
	* config/rs6000/476.opt (mpreserve-link-stack): Mark as Undocumented.
	It seems to be an internal option that is enabled by default on the
	cpu that can benefit from it.