docs: Adjust -Wimplicit-fallthrough= documentation for C23

I've noticed in -Wimplicit-fallthrough= documentation we talk about
[[fallthrough]]; for C++17 but don't mention that it is also standard
way to suppress the warning for C23.

2025-09-16  Jakub Jelinek  <jakub@redhat.com>

	* doc/invoke.texi (Wimplicit-fallthrough=): Document that also C23
	provides a standard way to suppress the warning with [[fallthrough]];.

(cherry picked from commit 5eb86c29d29fa25f8ad81d75730a993bb03fb558)