Doc: Copy-edit text about -Wno-xxx [PR90468]

Doc: Copy-edit text about -Wno-xxx [PR90468]

The issue is specifically about a missing word, but I spotted other
copy-editing issues like misplaced hyphens in nearby text.  I also
thought that the -Wimplicit example was anachronistic because it's a
hard error in modern C dialects rather than a warning, and replaced it
with something users are more likely to run into.

gcc/ChangeLog
	PR c++/90468
	* doc/invoke.texi (Warning Options): Clean up text describing
	-Wno-xxx.