message: fix(docs): explain PubSub.v1 property
diff: */
  /**
  + * Reference to internal generated clients, advanced use only.
  + *
  * @name PubSub.v1
  * @see v1.PublisherClient
  * @see v1.SubscriberClient
  */
  /**
  + * Reference to internal generated clients, advanced use only.
  + *
  * @name module:@google-cloud/pubsub.v1
  * @see v1.PublisherClient
  * @see v1.SubscriberClient
  