6 files declare their use of Transaction

Connection.php in core/tests/fixtures/database_drivers/custom/fake/Connection.php
DriverSpecificTransactionTestBase.php in core/tests/Drupal/KernelTests/Core/Database/DriverSpecificTransactionTestBase.php
StubConnection.php in core/tests/Drupal/Tests/Core/Database/Stub/StubConnection.php
Transaction.php in core/tests/fixtures/database_drivers/module/core_fake/src/Driver/Database/CoreFakeWithAllCustomClasses/Transaction.php
TransactionManagerBase.php in core/lib/Drupal/Core/Database/Transaction/TransactionManagerBase.php
TransactionManagerInterface.php in core/lib/Drupal/Core/Database/Transaction/TransactionManagerInterface.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.