Can't reference labels in top-comments

Both references in this example won't resolve:

```ocaml
module M : sig
  (** {1:label Heading in top-comment} *)
end

module type MT = sig
  (** {1:label Heading in top-comment} *)
end

(** {!M.label}
    {!MT.label} *)
```

They'll resolve if something is added before both comments.