2 files declare their use of AssertMailTrait

ConfigTranslationUiTestBase.php in core/modules/config_translation/tests/src/Functional/ConfigTranslationUiTestBase.php
MailCaptureTest.php in core/tests/Drupal/FunctionalTests/MailCaptureTest.php

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