Update example date in preamble commentary.

Update example date in preamble commentary.
(time-stamp): Fix parsing of "%%a" in time-stamp-pattern (change
regexp subpattern 5)
(time-stamp-pattern): Initialize to nil to avoid regexp work in
default case.
(time-stamp-string): Call set-time-zone-rule instead of setenv
(time-stamp-hhmmss): Remove (not needed after all).
(time-stamp-month-dd-yyyy, time-stamp-dd/mm/yyyy,
time-stamp-mon-dd-yyyy, time-stamp-dd-mon-yy, time-stamp-yy/mm/dd,
time-stamp-yyyy/mm/dd, time-stamp-yyyy-mm-dd, time-stamp-yymmdd,
time-stamp-hh:mm:ss, time-stamp-hhmm): Make obsolete.
(time-stamp-format, time-stamp-pattern, time-stamp-warn-inactive,
time-stamp-inserts-lines, time-stamp-count, time-stamp-time-zone,
time-stamp, time-stamp-toggle-active, time-stamp-string-preprocess,
time-stamp-once, time-stamp-conv-warn): Tweak doc strings.