diff --git a/flask/helpers.py b/flask/helpers.py
index 37843a4c..c0495906 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -251,7 +251,8 @@ def url_for(endpoint, **values):
       parameter must be set to `True` or a `ValueError` is raised. The default
       behavior uses the same scheme as the current request, or
       ``PREFERRED_URL_SCHEME`` from the :ref:`app configuration <config>` if no
-      request context is available.
+      request context is available. As of Werkzeug 0.10, this also can be set
+      to an empty string to build protocol-relative URLs.
     :param _anchor: if provided this is added as anchor to the URL.
     :param _method: if provided this explicitly specifies an HTTP method.
     """