Fix time-stamp-time-zone bugs introduced in July

Fix time-stamp-time-zone bugs introduced in July

This fixes a bug introduced when the July changes to
format-time-string installed, as the changes were not
correctly handled in this module (Bug#22302).
Also, document time stamp time zones.
* lisp/time-stamp.el (time-stamp-time-zone): Document values better.
(time-stamp--format): New private function.
(time-stamp-string, time-stamp-string-preprocess)
(time-stamp-do-number): Use it.
* doc/emacs/files.texi (Time Stamps): Mention time zones.
* doc/misc/autotype.texi (Timestamps): Document time-stamp-time-zone.