1 files changed, 7 insertions, 3 deletions

diff --git a/lisp/diff.el b/lisp/diff.el
index e802048c31d..da6eb3d538c 100644
--- a/lisp/diff.el
+++ b/lisp/diff.el
@@ -80,10 +80,14 @@ were found."
 ;;;###autoload
 (defun diff (old new &optional switches no-async)
   "Find and display the differences between OLD and NEW files.
-Interactively the current buffer's file name is the default for NEW
-and a backup file for NEW is the default for OLD.
+When called interactively, read OLD and NEW using the minibuffer;
+the default for NEW is the current buffer's file name, and the
+default for OLD is a backup file for NEW, if one exists.
 If NO-ASYNC is non-nil, call diff synchronously.
-With prefix arg, prompt for diff switches."
+
+When called interactively with a prefix argument, prompt
+interactively for diff switches.  Otherwise, the switches
+specified in `diff-switches' are passed to the diff command."
   (interactive
    (let (oldf newf)
      (setq newf (buffer-file-name)