trivial: Fix up some gtk-doc markup to not explode for old versions
trivial: Fix up some gtk-doc markup to not explode for old versions

ximion/appstream-glib,ikeydoherty/appstream-glib,superm1/appstream-glib,ximion/appstream-glib,kalev/appstream-glib,ikeydoherty/appstream-glib,superm1/appstream-glib,kalev/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,mitya57/appstream-glib,hughsie/appstream-glib,mitya57/appstream-glib