Stop generating unattached documentation comments in Ocaml

[tperami]

In the OCaml generation, Ott sometime generates documentation comments starting with (** in places where they are not documenting any particular item:

(** auxiliary functions on the new list types *)
(** library functions *)
(** subrules *)
(** auxiliary functions *)
(** free variables *)
(** substitutions *)
(** context application *)

This in turn generate the warning 50 of ocaml compiler:

Warning 50: unattached documentation comment (ignored)

It would be nice to change those generated comments to normal OCaml comments starting with a single *.