Gnus: Improve subthread sorting;

Gnus: Improve subthread sorting;
 make subthread sorting customizable and add docs for it

* doc/misc/gnus.texi (Sorting the Summary Buffer): Document new defcustom
`gnus-subthread-sort-functions' and remove the obsolete documentation
of `gnus-sort-threads-recursively'.

* lisp/gnus/gnus-sum.el (gnus-subthread-sort-functions): New defcustom.
(gnus-sort-threads-recursively): Delete defcustom.
(gnus-sort-threads-recursive): Adapt accordingly.

* lisp/gnus/gnus-sum.el (gnus-sort-subthreads-recursive): New function.
(gnus-sort-threads-recursive): Use it.
(gnus-sort-threads): Unconditionally call `gnus-sort-threads-recursive'
again.  Now that determines how to sort subthreads.