bug#33870: 27.0.50; xref-goto-xref not configurable

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Thu, 03 Jan 2019 02:18:50 +0200
> Cc: 33870@debbugs.gnu.org, Dmitry Gutov <dgutov@yandex.ru>
> 
> 1. simplifies ‘xref--show-pos-in-buf’ while at the same time
>    preserves the current behavior and respects user's customization
>    of display actions;
> 
> 2. makes the xref buffer non-obtrusive like *Completions*
>    in xref--show-xref-buffer;
> 
> 3. turns the existing arg QUIT of xref-goto-xref into a prefix arg,
>    so a natural key sequence ‘C-u RET’ will quit the window.
>    This is similar to the prefix arg of quit-window.

Please be sure to document any user-visible behavior changes in NEWS
and in the manual.

Thanks.