[binutils-gdb] S390: Document guarded-storage register support

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 289e23aa03084b22c73ebdcf18371f1e6666ead0 ***

Author: Andreas Arnez <arnez@linux.vnet.ibm.com>
Branch: master
Commit: 289e23aa03084b22c73ebdcf18371f1e6666ead0

S390: Document guarded-storage register support

This documents the newly added support for guarded-storage registers on
IBM z.

gdb/ChangeLog:

	* NEWS: Advertise support for guarded-storage registers on IBM z.

gdb/doc/ChangeLog:

	* gdb.texinfo (S/390 and System z Features): Document the new
	features org.gnu.gdb.s390.gs and org.gnu.gdb.s390.gsbc.