explicitly say what the difference is between retired and actual

AUTHORS

@@ -1,3 +1,8 @@
+E-mail addresses listed here are not intended for support, please see
+the `support section`_ instead.
+
+.. support section: https://borgbackup.readthedocs.io/en/stable/support.html
+
 Borg authors ("The Borg Collective")
 ------------------------------------