I notice that a reference to GtkStrengthBar had been left in this file
when it was renamed to GtkLevelBar. On closer inspection, it looks like
it's been unused for 13 years. To quote docs/reference/ChangeLog:
> 2000-10-23 Havoc Pennington <hp@redhat.com>
>
> [...] Removed objects_grouped.sgml from gtk-docs.sgml for now, it
> doesn't seem useful anymore.
https://bugzilla.gnome.org/show_bug.cgi?id=700575