;; BUFFER is nil. If ASYNC is non-nil, run asynchronously. If REV1
;; and REV2 are non-nil, report differences from REV1 to REV2. If
;; REV1 is nil, use the working revision (as found in the
-;; repository) as the older revision; if REV2 is nil, use the
+;; repository) as the older revision if REV2 is nil as well;
+;; otherwise, diff against an empty tree. If REV2 is nil, use the
;; current working-copy contents as the newer revision. This
;; function should pass the value of (vc-switches BACKEND 'diff) to
;; the backend command. It should return a status of either 0 (no