Add @file documentation to menu-region--footer.html.twig template

Closes #3443869

core/modules/navigation/templates/menu-region--footer.html.twig

+{#
+/**
+ * @file
+ * Default theme implementation to display the navigation footer menu.
+ *
+ * Available variables:
+ * - menu_name: The machine name of the user menu.
+ * - help: TRUE if "Help" module is enabled.
+ * - title: A name of account.
+ * - items: A nested list of menu items. Each menu item contains:
+ *   - attributes: HTML attributes for the menu item.
+ *   - below: The menu item child items.
+ *   - title: The menu link title.
+ *   - url: The menu link URL, instance of \Drupal\Core\Url
+ *   - localized_options: Menu link localized options.
+ *   - is_expanded: TRUE if the link has visible children within the current
+ *     menu tree.
+ *   - is_collapsed: TRUE if the link has children within the current menu tree
+ *     that are not currently visible.
+ *   - in_active_trail: TRUE if the link is in the active trail.
+ *
+ * @ingroup themeable
+ */
+#}
 {% set menu_heading_id = 'menu--' ~ menu_name|clean_unique_id %}
 <div class="admin-toolbar__item">
   <h4 id="{{ menu_heading_id }}" class="toolbar-block__title visually-hidden focusable">{{ title }}</h4>