storage: explain flat bucket namespace

Fixes #863

Author: stephenplusplus

lib/storage/index.js

@@ -178,6 +178,10 @@ Storage.prototype.bucket = function(name) {
 /**
  * Create a bucket.
  *
+ * Google Cloud Storage uses a flat namespace, so you can't create a bucket with
+ * a name that is already in use. For more information, see
+ * [Bucket Naming Guidelines](https://cloud.google.com/storage/docs/bucketnaming.html#requirements).
+ *
  * @resource [Buckets: insert API Documentation]{@link https://cloud.google.com/storage/docs/json_api/v1/buckets/insert}
  * @resource [Durable Reduced Availability]{@link https://cloud.google.com/storage/docs/durable-reduced-availability}
  * @resource [Nearline]{@link https://cloud.google.com/storage/docs/nearline}