C++: Add opindex for -Wchanges-meaning [PR117157]

Adds missing opindex for -Wchanges-meaning

Pushed as obvious after building the HTML and checking the index.

gcc/ChangeLog:

	PR c++/117157
	* doc/invoke.texi (Wno-changes-meaning): Add opindex.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>