diff --git a/synapse/http/server.py b/synapse/http/server.py
index 76c561d10598c34890c17f8c21d35e88b307db15..0dbdce2839df033e335348145e6b32e8e48508fa 100644
--- a/synapse/http/server.py
+++ b/synapse/http/server.py
@@ -79,6 +79,13 @@ class JsonResource(HttpServer, resource.Resource):
     Resources.
 
     Register callbacks via register_path()
+
+    Callbacks can return a tuple of status code and a dict in which case the
+    the dict will automatically be sent to the client as a JSON object.
+
+    The JsonResource is primarily intended for returning JSON, but callbacks
+    may send something other than JSON, they may do so by using the methods
+    on the request object and instead returning None.
     """
 
     isLeaf = True