diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index c088eed..5bf2967 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -15,6 +15,13 @@
 
 /**
  * Discovers available extensions in the filesystem.
+ *
+ * To also discover test modules, add
+ * @code
+ * $settings['extension_discovery_scan_tests'] = TRUE;
+ * @encode
+ * to your settings.php.
+ *
  */
 class ExtensionDiscovery {
 
@@ -134,6 +141,12 @@ public function __construct($root, $use_file_cache = TRUE, $profile_directories
    * - the site-wide directory; i.e., /
    * - the site-specific directory; e.g., /sites/example.com
    *
+   * To also find test modules, add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @encode
+   * to settings.php.
+   *
    * The information is returned in an associative array, keyed by the extension
    * name (without .info.yml extension). Extensions found later in the search
    * will take precedence over extensions found earlier - unless they are not
diff --git a/core/lib/Drupal/Core/Extension/ModuleInstallerInterface.php b/core/lib/Drupal/Core/Extension/ModuleInstallerInterface.php
index bd2923a..048b9e6 100644
--- a/core/lib/Drupal/Core/Extension/ModuleInstallerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ModuleInstallerInterface.php
@@ -23,6 +23,12 @@
    *   - Invoke hook_install() and add it to the list of installed modules.
    * - Invoke hook_modules_installed().
    *
+   * To install test modules add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @encode
+   * to your settings.php.
+   *
    * @param string[] $module_list
    *   An array of module names.
    * @param bool $enable_dependencies
diff --git a/core/modules/simpletest/src/KernelTestBase.php b/core/modules/simpletest/src/KernelTestBase.php
index 6210396..64ac2f0 100644
--- a/core/modules/simpletest/src/KernelTestBase.php
+++ b/core/modules/simpletest/src/KernelTestBase.php
@@ -485,6 +485,12 @@ protected function installEntitySchema($entity_type_id) {
   /**
    * Enables modules for this test.
    *
+   * To enable test modules outside of the testing environment, add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @encode
+   * to your settings.php.
+   *
    * @param array $modules
    *   A list of modules to enable. Dependencies are not resolved; i.e.,
    *   multiple modules have to be specified with dependent modules first.
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 1e3bfdb..ca53880 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1010,6 +1010,12 @@ protected function initKernel(Request $request) {
   /**
    * Install modules defined by `static::$modules`.
    *
+   * To enable test modules outside of the testing environment, add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @encode
+   * to your settings.php.
+   *
    * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
    *   The container.
    */
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index 0c2db9a..a724702 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -786,6 +786,12 @@ protected function installEntitySchema($entity_type_id) {
   /**
    * Enables modules for this test.
    *
+   * To enable test modules outside of the testing environment, add
+   * @code
+   * $settings['extension_discovery_scan_tests'] = TRUE;
+   * @encode
+   * to your settings.php.
+   *
    * @param string[] $modules
    *   A list of modules to enable. Dependencies are not resolved; i.e.,
    *   multiple modules have to be specified individually. The modules are only