Update document

Author: hrchu

confluent_kafka/src/Consumer.c

@@ -1236,7 +1236,8 @@ static PyMethodDef Consumer_methods[] = {
           "  :param bool cached: Instead of querying the broker used cached information. "
           "Cached values: The low offset is updated periodically (if statistics.interval.ms is set) while "
           "the high offset is updated on each message fetched from the broker for this partition.\n"
-          "  :returns: Tuple of (low,high) on success or None on timeout.\n"
+          "  :returns: Tuple of (low,high) on success or None on timeout. "
+          "The high offset is the offset of the last message + 1.\n"
           "  :rtype: tuple(int,int)\n"
           "  :raises: KafkaException\n"
           "  :raises: RuntimeError if called on a closed consumer\n"