Doc: Document -Wpsabi [PR81831]

Doc: Document -Wpsabi [PR81831]

Per the issue, there were a couple places in the manual where
-Wno-psabi was mentioned, but the option itself was not documented.

gcc/c-family/ChangeLog
	PR c/81831
	* c.opt (Wpsabi): Remove "Undocumented" modifier and add a
	documentation string.

gcc/ChangeLog
	PR c/81831
	* doc/invoke.texi (Option Summary): Add -Wno-psabi.
	(Warning Options): Document -Wpsabi separately from -Wabi.
	Note it's enabled by default, not just implied by -Wabi.
	Replace the detailed example for a GCC 4.4 change for x86
	(which is unlikely to be very interesting nowadays) with
	just a list of all targets that presently diagnose these
	warnings.
	(RS/6000 and PowerPC Options): Add cross-references for
	-Wno-psabi.