improved docs about goBack()

framework/web/Controller.php

@@ -172,6 +172,8 @@ class Controller extends \yii\base\Controller
      * return $this->goBack();
      * ```
      *
+     * For this function to work you have to [[User::setReturnUrl()|set the return URL]] in appropriate places before.
+     *
      * @param string|array $defaultUrl the default return URL in case it was not set previously.
      * If this is null and the return URL was not set previously, [[Application::homeUrl]] will be redirected to.
      * Please refer to [[User::setReturnUrl()]] on accepted format of the URL.