diff --git a/core/modules/user/lib/Drupal/user/Tests/UserAutocompleteTest.php b/core/modules/user/lib/Drupal/user/Tests/UserAutocompleteTest.php
index eda2685..5679b18 100644
--- a/core/modules/user/lib/Drupal/user/Tests/UserAutocompleteTest.php
+++ b/core/modules/user/lib/Drupal/user/Tests/UserAutocompleteTest.php
@@ -48,7 +48,7 @@ function testUserAutocomplete() {
     $this->assertRaw($this->unprivileged_user->name, 'User name found in autocompletion results.');
 
     // Test that anonymous username is in the result.
-    $anonymous_name = $this->randomName();
+    $anonymous_name = $this->randomString();
     config('user.settings')->set('anonymous', $anonymous_name)->save();
     $this->drupalGet('user/autocomplete', array('query' => array('q' => drupal_substr($anonymous_name, 0, 4), 'anonymous' => '1')));
     $this->assertRaw($anonymous_name, 'The anonymous name found in autocompletion results.');
diff --git a/core/modules/user/user.pages.inc b/core/modules/user/user.pages.inc
index 9fa30e3..b1473ef 100644
--- a/core/modules/user/user.pages.inc
+++ b/core/modules/user/user.pages.inc
@@ -14,6 +14,15 @@
 /**
  * Menu callback for user autocompletion.
  *
+ * Like other autocomplete functions, this function inspects the 'q' query
+ * parameter for the string to use to search for suggestions. If the name used
+ * to indicate anonymous users (e.g. "Anonymous") is to be included as a
+ * possible suggestion, the 'anonymous' query parameter should be set
+ * additionally. For example, http://example.com/user/autocomplete?q=An might
+ * return "Andrew" and "Anne", while
+ * http://example.com/user/autocomplete?q=An&anonymous=1 will additionally
+ * return "Anonymous".
+ *
  * @return \Symfony\Component\HttpFoundation\JsonResponse
  *   A JSON response containing the autocomplete suggestions for existing users.
  */