diff --git a/lisp/mail/rmail.el b/lisp/mail/rmail.el
index b6c36c2f76c..5747091c498 100644
--- a/lisp/mail/rmail.el
+++ b/lisp/mail/rmail.el
@@ -2684,7 +2684,9 @@ N defaults to the current message."
 	  (and (string-match text-regexp content-type-header) t)))))
 
 (defcustom rmail-show-message-verbose-min 200000
-  "Message size at which to show progress messages for displaying it."
+  "Message size at which to show progress messages for displaying it.
+Messages longer than this (in characters) will produce echo-area
+messages when Rmail processes such a message for display."
   :type 'integer
   :group 'rmail
   :version "23.1")