Issue #3441727 by quietone: Document update fixture drupal.broken.xml

(cherry picked from commit 24dfa8d486)

core/modules/update/tests/fixtures/release-history/drupal.broken.xml

@@ -1,3 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
+<!--
+This XML file is empty to test no information scenarios.
+-->
 <project xmlns:dc="http://purl.org/dc/elements/1.1/">
 </project>

core/modules/update/tests/src/Functional/UpdateSemverCoreTest.php

@@ -77,6 +77,11 @@ class UpdateSemverCoreTest extends UpdateSemverCoreTestBase {
 
   /**
    * Checks that Drupal recovers after problems connecting to update server.
+   *
+   * This test uses the following XML fixtures.
+   *  - drupal.broken.xml
+   *  - drupal.sec.8.0.2.xml
+   *     'supported_branches' is '8.0.,8.1.'.
    */
   public function testBrokenThenFixedUpdates() {
     $this->drupalLogin($this->drupalCreateUser([