diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 9eac5b26f03..eba5be9cbec 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1009,6 +1009,8 @@ or BRANCH^ (where \"^\" can be repeated)."
       (buffer-string))))
 
 (defun vc-git-region-history (file buffer lfrom lto)
+  "Insert into BUFFER the history of FILE for lines LFROM to LTO.
+This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
   ;; The "git log" command below interprets the line numbers as applying
   ;; to the HEAD version of the file, not to the current state of the file.
   ;; So we need to look at all the local changes and adjust lfrom/lto