diff --git a/lisp/vc/diff.el b/lisp/vc/diff.el
index edb231cf605..469888078c4 100644
--- a/lisp/vc/diff.el
+++ b/lisp/vc/diff.el
@@ -148,7 +148,8 @@ Possible values are:
   "Compare the OLD and NEW file/buffer.
 If the optional SWITCHES is nil, the switches specified in the
 variable ‘diff-switches’ are passed to the diff command,
-otherwise SWITCHES is used.
+otherwise SWITCHES is used.  SWITCHES can be a string or a list
+of strings.
 
 If NO-ASYNC is non-nil, call diff synchronously.