bug#32931: time-stamp-format: offer numeric time zones too

[Stephen Gildea <stepheng+emacs@gildea.com>]

The time-stamp package has unusual compatibility issues that govern
when code changes are appropriate to document.  In the source, it is
noted that

;;;      At all times, all the formats recommended in the doc string
;;; of time-stamp-format will work not only in the current version of
;;; Emacs, but in all versions that have been released within the past
;;; two years.

The reason for this promise is that it is common for people to set
time-stamp-pattern or time-stamp-format in a local-variables section of
a file.  Files so annotated may be edited by different people and with
different versions of Emacs.  So that Emacs behaves consistently, the
time-stamp package recommends only formats that behave the same way
across all versions likely to be in simultaneous use.

In September I updated the documentation for changes implemented
previously; that commit included a section in NEWS.  I will eventually
document the change that sparked this discussion, too, and I'll add it
to NEWS at the same time.

So while I am aware of my responsibility to document, I am also
mindful of my responsibility to not document yet.

 < Stephen