docs: ApiFuture<Void> rollback() {
      pending = false;
  
      RollbackRequest.Builder reqBuilder = RollbackRequest.newBuilder();
      reqBuilder.setTransaction(transactionId);
      reqBuilder.setDatabase(firestore.getDatabaseName());
  
      ApiFuture<Empty> rollbackFuture =
          firestore.sendRequest(reqBuilder.build(), firestore.getClient().rollbackCallable());
  
      return ApiFutures.transform(
          rollbackFuture,
          new ApiFunction<Empty, Void>() {
            @Override
            public Void apply(Empty beginTransactionResponse) {
              return null;
            }
          });
    }