Mention the vc diff switches in diff-switches

* lisp/vc/diff.el (diff-switches): Mention the vc diff switches in
the doc string (bug#4422).

1 files changed, 6 insertions, 1 deletions

diff --git a/lisp/vc/diff.el b/lisp/vc/diff.el
index 5fa771f5f1e..9ece8bc1fb4 100644
--- a/lisp/vc/diff.el
+++ b/lisp/vc/diff.el
@@ -39,7 +39,12 @@
 
 ;;;###autoload
 (defcustom diff-switches (purecopy "-u")
-  "A string or list of strings specifying switches to be passed to diff."
+  "A string or list of strings specifying switches to be passed to diff.
+
+This variable is also used in the `vc-diff' command (and related
+commands) if the backend-specific diff switch variable isn't
+set (`vc-git-diff-switches' for git, for instance), and
+`vc-diff-switches' isn't set."
   :type '(choice string (repeat string))
   :group 'diff)