This is because gtk-doc seems to require an instantiable type to recognize the types, because else base_init() won't be called. Should be possible to add a work-around here, maybe link scandoc to an additional helper lib.
Fixed with revision 20531. 2006-03-24 Benedikt Meurer <benny@xfce.org> * docs/reference/thunarx/tmpl/thunarx-file-info.sgml, thunarx/thunarx-file-info.c: Register the "changed" and "renamed" signals right after the type is registered. This way gtk-doc will properly display the signals (gtk-doc doesn't invoke base_init() for interfaces). Bug #1418.