explain event emitter usage.

lib/pubsub/subscription.js

@@ -53,8 +53,10 @@ var util = require('../common/util.js');
  *
  * - {@linkcode module:pubsub/topic#subscription}
  *
- * A subscription won't begin pulling for messages until you register an event
- * listener. See below for an example.
+ * All Subscription objects are instances of an
+ * [EventEmitter]{@link http://nodejs.org/api/events.html}. The subscription
+ * will pull for messages automatically as long as there is at least one
+ * listener assigned for the `message` event.
  *
  * @alias pubsub/subscription
  * @constructor
@@ -79,7 +81,7 @@ var util = require('../common/util.js');
  * // From {@linkcode module:pubsub/topic#subscribe}:
  * //-
  * var topic = pubsub.topic('my-existing-topic');
- * topic.subscribe('new-subscrpition', function(err, subscription) {
+ * topic.subscribe('new-subscription', function(err, subscription) {
  *   // `subscription` is a Subscription object.
  * });
  *