menu binding docs: Link to insert_action_group()

menu binding docs: Link to insert_action_group()

It's pretty useless to make a menu without actions behind it and people
who are using gtk_menu_shell_bind_model() directly are probably not
interested in doing it the GtkApplicationWindow way (so they won't get
the "app" and "win" groups for free).  People are going to need to call
gtk_widget_insert_action_group(), so mention this in the docs to help
them along.