[Document] Fix wrong return value of transaction.commit and .rollback

lib/transaction.js

@@ -122,7 +122,7 @@ Transaction.LOCK = Transaction.prototype.LOCK = {
 /**
  * Commit the transaction
  *
- * @return {this}
+ * @return {Promise}
  */
 Transaction.prototype.commit = function() {
   var self = this;
@@ -145,7 +145,7 @@ Transaction.prototype.commit = function() {
 /**
  * Rollback (abort) the transaction
  *
- * @return {this}
+ * @return {Promise}
  */
 Transaction.prototype.rollback = function() {
   var self = this;