Docs: Mention SERVER_NAME in the url_for() docstring

flask/helpers.py

@@ -305,7 +305,9 @@ def url_for(endpoint, **values):
 
     :param endpoint: the endpoint of the URL (name of the function)
     :param values: the variable arguments of the URL rule
-    :param _external: if set to `True`, an absolute URL is generated.
+    :param _external: if set to `True`, an absolute URL is generated. Server
+      address can be changed via `SERVER_NAME` configuration variable which
+      defaults to `localhost`.
     :param _anchor: if provided this is added as anchor to the URL.
     :param _method: if provided this explicitly specifies an HTTP method.
     """