diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index b09df75..8d8fe6a 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -22,7 +22,8 @@
  *
  * The module/hook system is functional and operates on a fixed module list.
  * Additional modules needed in a test may be loaded and added to the fixed
- * module list.
+ * module list. Do not use module_enable() because it does not upgrade the
+ * fixed module list.
  *
  * @see DrupalUnitTestBase::$modules
  * @see DrupalUnitTestBase::enableModules()
@@ -205,7 +206,8 @@ protected function installSchema($module, $table) {
    * @param bool $install
    *   (optional) Whether to install the list of modules via module_enable().
    *   Defaults to TRUE. If FALSE, the new modules are only added to the fixed
-   *   module list and loaded.
+   *   module list and loaded. Services defined by the module is only loaded
+   *   when set to TRUE.
    *
    * @todo Remove this method as soon as there is an Extensions service
    *   implementation that is able to manage a fixed module list.