Update reference for threading RFC.

lisp/mh-e/mh-thread.el

@@ -27,9 +27,11 @@
 ;; The threading portion of this files tries to implement the
 ;; algorithm described at:
 ;;   http://www.jwz.org/doc/threading.html
-;; It also begins to implement the IMAP Threading extension RFC. The
-;; implementation lacks the reference and subject canonicalization of
-;; the RFC.
+;; It also begins to implement the threading section of the IMAP -
+;; SORT and THREAD Extensions RFC at:
+;;   http://tools.ietf.org/html/rfc5256
+;; The implementation lacks the reference and subject canonicalization
+;; of the RFC.
 
 ;; In the presentation buffer, children messages are shown indented
 ;; with either [ ] or < > around them. Square brackets ([ ]) denote