Mention Ajax loading filter in docs.

doc/introduction.rst

@@ -267,6 +267,13 @@ When your forms contain foreign keys, have those **related models loaded via aja
         }
     }
 
+To filter the results that are loaded via ajax, you can use::
+
+    form_ajax_refs = {
+        'active_user': QueryAjaxModelLoader('user', db.session, User,
+                                     filters=["is_active=True", "id>1000"])
+    }
+
 To **manage related models inline**::
 
     inline_models = ['post', ]