[docs] note that -g opts are implicitly negatable too

[docs] note that -g opts are implicitly negatable too

Back in 2017, I made -g* options implicitly negatable, without
realizing there was documentation that required updating.  Oops.
Fixed, at last!

for gcc/ChangeLog

	* doc/options.texi (option, RejectNegative): Mention that
	-g-started options are also implicitly negatable.