Issue #2538158 by joyceg: Clarification to where hook_modules_installed should be implemented

core/lib/Drupal/Core/Extension/module.api.php

@@ -125,6 +125,8 @@ function hook_module_preinstall($module) {
  * \Drupal\Core\Extension\ModuleHandler::install() for a detailed description of
  * the order in which install hooks are invoked.
  *
+ * This hook should be implemented in a .module file, not in an .install file.
+ *
  * @param $modules
  *   An array of the modules that were installed.
  *