Add some missing @opindex entries [PR122243]

Add some missing @opindex entries [PR122243]

The options handled in this patch already have documentation but are
either missing an @opindex entry entirely, or index only the negative
option form.

gcc/ChangeLog
	PR other/122243
	* doc/cppopts.texi (fcanonical-system-headers): Add @opindex.
	* doc/invoke.texi (fdump-ada-spec-slim): Add @opindex.
	(fcontract-semantic): Likewise.
	(fdiagnostics-plain-output): Likewise.
	(Wc11-c2x-compat): Likewise.
	(Wvla-parameter): Likewise.
	(fanalyzer-verbose-edges): Likewise.
	(fanalyzer-verbose-state-changes): Likewise.
	(fanalyzer-verbosity): Likewise.
	(flimit-function-alignment): Likewise.