message: docs(subscription): add mention of auto-leasing
diff: @@ -64,6 +64,11 @@ var MAX_ACK_IDS_PER_REQUEST = 3000;
  * time. You can fine tune this value by adjusting the
  * `options.flowControl.maxMessages` option.
  *
  + * Subscription objects handle ack management, by automatically extending the
  + * ack deadline while the message is being processed, to then issue the ack or
  + * nack of such message when the processing is done. **Note:** message
  + * redelivery is still possible.
  + *
  * @class
  *
  * @param {PubSub} pubsub PubSub object.
  