bug#32849: 26.1; xref-marker-ring-length user option doesn't have setter

[Eli Zaretskii]

> From: Allen Li <darkfeline@felesatra.moe>
> Date: Thu, 8 Nov 2018 23:01:21 -0800
> Cc: 32849@debbugs.gnu.org
> 
> On Sat, Oct 27, 2018 at 2:57 AM Eli Zaretskii <eliz@gnu.org> wrote:
> > Thanks.  Please see a few minor comments below.
> 
> Thanks for your comments.  I have attached new patches based off master.

Thanks, I pushed this to the master branch.

One nit:

> * doc/lispref/sequences.texi (Rings): Document new function
> * etc/NEWS: Document new function
> * lisp/emacs-lisp/ring.el (ring-resize): New function
> * test/lisp/emacs-lisp/ring-tests.el (ring-test-ring-resize): New tests

Please end each entry in the commit log with a period, as it's
supposed to be one or more complete sentences.