diff -u b/core/modules/update/tests/modules/update_test/update_test.module b/core/modules/update/tests/modules/update_test/update_test.module
--- b/core/modules/update/tests/modules/update_test/update_test.module
+++ b/core/modules/update/tests/modules/update_test/update_test.module
@@ -100,6 +100,11 @@
  *   The project short name the update manager is trying to fetch data for (the
  *   fetch URLs are of the form: [base_url]/[project_name]/[core_version]).
  *
+ * @return StreamedResponse|Response
+ *   A StreamedResponse object containing the content of the XML release file
+ *   for the specified project if one is available; a Response object with no
+ *   content otherwise.
+ *
  * @see update_test_menu()
  */
 function update_test_mock_page($project_name) {