docs: pubsub delete topic

In v1beta2, deleting a topic does *not* delete corresponding subscriptions

"Deletes the topic with the given name. Returns NOT_FOUND if the topic does
 not exist. After a topic is deleted, a new topic may be created with the
 same name; this is an entirely new topic with none of the old configuration
 or subscriptions. Existing subscriptions to this topic are not deleted."

source: <https://cloud.google.com/pubsub/reference/rest/v1beta2/projects/topics/delete>

lib/pubsub/topic.js

@@ -206,7 +206,7 @@ Topic.prototype.publish = function(messages, callback) {
 };
 
 /**
- * Delete the topic. This will also delete any subscriptions to this topic.
+ * Delete the topic. This will not delete subscriptions to this topic.
  *
  * @param {function=} callback - The callback function.
  *