docs: publicly document Datastore.dataset - fixes googleapis#547

Author: stephenplusplus

lib/datastore/index.js

@@ -99,6 +99,16 @@ Datastore.dataset = Dataset;
  * @param  {object=} options - Configuration object.
  * @return {module:datastore/dataset}
  */
+/**
+ * Create a Dataset object to reference an existing dataset.
+ *
+ * @param {object=} options - [Configuration object](#/docs/?method=gcloud).
+ * @param {string=} options.apiEndpoint - Override the default API endpoint used
+ *     to reach Datastore. This is useful for connecting to your local Datastore
+ *     server (usually "http://localhost:8080").
+ * @param {string} options.namespace - Namespace to isolate transactions to.
+ * @return {module:datastore/dataset}
+ */
 Datastore.prototype.dataset = function(options) {
   options = options || {};
   // Mix in global config data to the provided options.