Merge pull request #1144 from untitaker/url_for_scheme_docs

Add documentation about url_for's default scheme

flask/helpers.py

@@ -248,7 +248,10 @@ def url_for(endpoint, **values):
       address can be changed via `SERVER_NAME` configuration variable which
       defaults to `localhost`.
     :param _scheme: a string specifying the desired URL scheme. The `_external`
-      parameter must be set to `True` or a `ValueError` is raised.
+      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.
     :param _anchor: if provided this is added as anchor to the URL.
     :param _method: if provided this explicitly specifies an HTTP method.
     """