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

[Juri Linkov]

>> 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.

I don't want to advertise this fix because the primary way to get back
for xref should be ‘M-,’.  Setting the mark is only needed for users
who expect that every significant change in position should leave a trace
in the global mark ring.