Index: docs/topics/testing.txt
===================================================================
--- docs/topics/testing.txt	(revision 16553)
+++ docs/topics/testing.txt	(working copy)
@@ -710,6 +710,17 @@
         details view, which is a good way to test code paths that use the
         :meth:`django.http.HttpRequest.is_ajax()` method.
 
+        .. admonition:: CGI specification
+
+            The headers sent via ``**extra`` should follow CGI_ specification. For
+            example, A "Host" header specified in the HTTP request sent from the
+            browser to the server should be passed as ``HTTP_HOST``, which is 
+            where ``django.core.handlers.wsgi.WSGIRequest.get_host`` looks to 
+            determine the host serving the request. 
+
+            .. _CGI: http://www.w3.org/CGI/
+
+
         If you already have the GET arguments in URL-encoded form, you can
         use that encoding instead of using the data argument. For example,
         the previous GET request could also be posed as::