add documentation for stats_cb

Author: hqin

docs/index.rst

@@ -81,6 +81,8 @@ The Python bindings also provide some additional configuration properties:
 * ``error_cb(kafka.KafkaError)``: Callback for generic/global error events. This callback is served by
   poll().
 
+* ``stats_cb(json_str)``: Callback for statistics data. This callback is triggered by poll() every ``statistics.interval.ms`` (needs to be configured separately). Function argument ``json_str`` is a str instance of a JSON document containing statistics data.
+
 * ``on_delivery(kafka.KafkaError, kafka.Message)`` (**Producer**): value is a Python function reference
   that is called once for each produced message to indicate the final
   delivery result (success or failure).