doc: fix list of GNU extension date formats
I didn't check how long these were documented as GNU extensions,
nor when they were added by POSIX; but since they are all part
of POSIX 2008, we no longer need call them out as extensions.
The next version of POSIX will standardize %s:
http://austingroupbugs.net/view.php?id=169
but as that is not out yet, I didn't change %s.
* doc/coreutils.texi (Time conversion specifiers): %R and %z are
now POSIX.
(Date conversion specifiers): Likewise for %F, %g, %G.