document the register_settings_menu_item hook

docs/core_components/pages/editing_api.rst

@@ -553,6 +553,13 @@ The available hooks are:
     def register_frank_menu_item():
       return MenuItem('Frank', reverse('frank'), classnames='icon icon-folder-inverse', order=10000)
 
+.. _register_settings_menu_item:
+
+``register_settings_menu_item``
+  .. versionadded:: 0.6
+
+  As ``register_admin_menu_item``, but registers menu items into the 'Settings' sub-menu rather than the top-level menu.
+
 .. _construct_main_menu:
 
 ``construct_main_menu``