libstdc++: Document remaining C++20 implementation-defined behavior.

libstdc++: Document remaining C++20 implementation-defined behavior.

I have double checked that implementation-defined behavior in the [compliance]
(whether the implementation is freestanding) and [stringbuf.const] (initialization
of sequence pointers) are indeed null, and there are no corresponding entires in
earlier standards.

libstdc++-v3/ChangeLog:

	* doc/html/manual/status.html: Regenerate.
	* doc/xml/manual/status_cxx2020.xml: Add more entires.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>