diff --git a/lisp/button.el b/lisp/button.el
index 8dbb763281a..a6f70436f74 100644
--- a/lisp/button.el
+++ b/lisp/button.el
@@ -615,7 +615,9 @@ button at point is the button to describe."
 
 (defun button-buttonize (string callback &optional data)
   "Make STRING into a button and return it.
-When clicked, CALLBACK will be called with the optional DATA parameter."
+When clicked, CALLBACK will be called with the DATA as the
+function argument.  If DATA isn't present (or is nil), the button
+itself will be used instead as the function argument."
   (propertize string
               'face 'button
               'button t