c++: Move -Wdangling-reference to -Wextra [PR109642]

Sadly, -Wdangling-reference generates false positives for std::span-like
user classes, and it seems imprudent to attempt to improve the heuristic
in GCC 13.  Let's move the warning to -Wextra, that will hopefully
reduce the number of false positives the users have been seeing with 13.

I'm leaving the warning in -Wall in 14 where I think I can write code
to detect std::span-like classes.

	PR c++/109642
	PR c++/109640
	PR c++/109671

gcc/c-family/ChangeLog:

	* c.opt (Wdangling-reference): Move from -Wall to -Wextra.

gcc/ChangeLog:

	* doc/invoke.texi: Document that -Wdangling-reference is
	enabled by -Wextra.