docs(connection): clarify that Connection#transaction() promise res…

…olves to a command result

Fix #9919

lib/connection.js

@@ -469,7 +469,7 @@ Connection.prototype.startSession = _wrapConnHelper(function startSession(option
  * @method transaction
  * @param {Function} fn Function to execute in a transaction
  * @param {mongodb.TransactionOptions} [options] Optional settings for the transaction
- * @return {Promise<Any>} promise that resolves to the returned value of `fn`
+ * @return {Promise<Any>} promise that is fulfilled if Mongoose successfully committed the transaction, or rejects if the transaction was aborted or if Mongoose failed to commit the transaction. If fulfilled, the promise resolves to a MongoDB command result.
  * @api public
  */