bug#34908: Push mark in xref-push-marker-stack

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Mon, 18 Mar 2019 23:12:50 +0200
> Cc: joão távora <joaotavora@gmail.com>,
>       dmitry gutov <dgutov@yandex.ru>
> 
> Shouldn't xref-push-marker-stack push the mark like all normal commands do?

Fine with me to add that (I think etags.el-based M-. did this), but
please make sure the manual and NEWS are updated with this
information.

Thanks.