* doc/misc/gnus.texi (Summary Sorting): Mention
gnus-summary-sort-by-marks.
* lisp/gnus/gnus-sum.el (gnus-article-sort-by-marks): New
function (bug#23393).
(gnus-thread-sort-by-marks): Ditto.
(gnus-summary-sort-by-mark): New command suggested by Dan Jacobson.
(gnus-summary-mode-map): Add keystroke.
(gnus-summary-make-menu-bar): Add to menu.