diff --git a/lisp/ox-publish.el b/lisp/ox-publish.el
index 1e88667ac..ceada6345 100644
--- a/lisp/ox-publish.el
+++ b/lisp/ox-publish.el
@@ -358,7 +358,9 @@ You can overwrite this default per project in your
 ;;; Timestamp-related functions
 
 (defun org-publish-timestamp-filename (filename &optional pub-dir pub-func)
-  "Return path to timestamp file for filename FILENAME."
+  "Return path to timestamp file for filename FILENAME.
+The timestamp file name is constructed using FILENAME, publishing
+directory PUB-DIR, and PUB-FUNC publishing function."
   (setq filename (concat filename "::" (or pub-dir "") "::"
 			 (format "%s" (or pub-func ""))))
   (concat "X" (if (fboundp 'sha1) (sha1 filename) (md5 filename))))