bug#62229: 29.0.60; It's no longer possible to customize the xref ring size

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Date: Thu, 16 Mar 2023 18:11:06 -0400
> 
> 
> Emacs 28 had the function xref-set-marker-ring-length and the
> customizable variable xref-marker-ring-length which allowed growing
> the amount of history kept by xref.
> 
> This feature has been lost in Emacs 29; that variable has been
> obsoleted with no replacement.
> 
> It's not clear, looking at the change, whether this is because the
> size is now infinite or not.  The same commit that obsoleted that
> variable added the similar tags-location-ring-length.  However that
> variable has no docstring, no customize hooks, and setting it has no
> effect unless tags-reset-tags-tables is called.
> 
> At the very least, all this should be documented better, and the
> obsoleting of this function and variable should be mentioned in NEWS.
> If the history is indeed infinite now, that should be mentioned in
> NEWS as well.

Indeed, that change paid very little attention to documenting the
important aspects of adding Xref history and using it instead of a
limited-size ring.  We should do better.

I've now attempted to add the missing documentation; please take a
look at the latest emacs-29 branch.