trivial: Add missing gtk-doc
trivial: Add missing gtk-doc

hughsie/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,ikeydoherty/appstream-glib,ikeydoherty/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib