Discovered empirically. (#56)

Author: sureshvv

docs/api.rst

@@ -51,6 +51,10 @@ API
     If you for some reason already have the HTML you can jump straight
     to this method. Note, you still need to provide the URL where you
     got the HTML from so it can use that to download any external CSS.
+    
+  * When calling ``process_url()`` or ``process_html()``, you have to call ``process()``
+    at the end without arguments, in order to post process the pages that were
+    processed individually.
 
   The ``Processor`` instance will make two attributes available