Missing reference syntax for extension declarations

```ocaml
(* Consider this extensible type *)

type more = ..

type more +=
| At_the_door
| Behind_the_floor


(** It seems I can refer to {!more}, {!extension-At_the_door}
    {!extension-Behind_the_floor}. But not the point of extension
    (despite the fact that it has an anchor in the rendering (namely
    #extension-decl-At_the_door)). *)


```