Issue #83: QA remark: Explain why router rebuild is needed.

src/Event/UserRegisterRouteRedirectConfigSubscriber.php

@@ -41,7 +41,12 @@ public static function getSubscribedEvents(): array {
  }
 
  /**
- * Acts when oe_authentication.settings:redirect_user_register_route changes.
+ * Rebuilds route table on changing the user registration redirection.
+ *
+ * We need to rebuild the route table when an administrator changes the option
+ * to redirect the user registration form to EU Login. In normal operations
+ * the route table is only updated when the cache is being rebuilt, but the
+ * administrator expects this to take effect instantly.
  *
  * @param \Drupal\Core\Config\ConfigCrudEvent $event
  * The config CRUD event.