diff --git a/core/lib/Drupal/Core/Extension/module.api.php b/core/lib/Drupal/Core/Extension/module.api.php
index 6dda3cc..f07090b 100644
--- a/core/lib/Drupal/Core/Extension/module.api.php
+++ b/core/lib/Drupal/Core/Extension/module.api.php
@@ -888,6 +888,10 @@ function hook_updater_info_alter(&$updaters) {
  * Module dependencies do not belong to these installation requirements,
  * but should be defined in the module's .info.yml file.
  *
+ * During installation (when $phase == 'install'), if you need to load a class
+ * from your module, you'll need to either load the class include file directly.
+ * Both drupal_get_path() and module_load_include() will work.
+ *
  * The 'runtime' phase is not limited to pure installation requirements
  * but can also be used for more general status information like maintenance
  * tasks and security issues.