docs: datastore: document keys-only query

lib/datastore/request.js

@@ -503,6 +503,17 @@ DatastoreRequest.prototype.delete = function(keys, callback) {
  * //-
  * transaction.runQuery(query)
  *   .on('data', function (entity) {});
+ *
+ * //-
+ * // A keys-only query returns just the keys of the result entities instead of
+ * // the entities themselves, at lower latency and cost.
+ * //-
+ * var keysOnlyQuery = dataset.createQuery('Lion').select('__key__');
+ *
+ * transaction.runQuery(query, function(err, entities, endCursor, callback) {
+ *   // entities[].key = Key object
+ *   // entities[].data = Empty object
+ * });
  */
 DatastoreRequest.prototype.runQuery = function(q, callback) {
   var that = this;