diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index d6bd64f..e8fcee4 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -28,6 +28,8 @@
  *
  * @see \DrupalUnitTestBase::$modules
  * @see \DrupalUnitTestBase::enableModules()
+ *
+ * @ingroup testing
  */
 abstract class DrupalUnitTestBase extends UnitTestBase {
 
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
index 301c7b6..9d93fa0 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
@@ -17,8 +17,6 @@
  * function that needs the database will throw exceptions. These include
  * watchdog(), \Drupal::moduleHandler()->getImplementations(),
  * \Drupal::moduleHandler()->invokeAll() etc.
- *
- * @ingroup testing
  */
 abstract class UnitTestBase extends TestBase {
 
diff --git a/core/modules/system/core.api.php b/core/modules/system/core.api.php
index decdd48..1b87d9d 100644
--- a/core/modules/system/core.api.php
+++ b/core/modules/system/core.api.php
@@ -676,11 +676,11 @@
  *   WebTestBase uses a "testing" install profile, with a minimal set of
  *   modules enabled.
  * - For functional tests that do not test web output, define a class that
- *   extends \Drupal\simpletest\UnitTestBase. This class is much faster than
- *   WebTestBase, because instead of making a full install of Drupal, it uses
- *   an in-memory pseudo-installation. To use this test class, you will need
- *   to create the database tables you need and include the module files you
- *   need manually.
+ *   extends \Drupal\simpletest\DrupalUnitTestBase. This class is much faster
+ *   than WebTestBase, because instead of making a full install of Drupal, it
+ *   uses an in-memory pseudo-installation (similar to what the installer and
+ *   update scripts use). To use this test class, you will need to create the
+ *   database tables you need and install needed modules manually.
  * - The class name needs to end in the word Test.
  * - The namespace must be a subspace/subdirectory of \Drupal\yourmodule\Tests,
  *   where yourmodule is your module's machine name.