Describe the solution you'd like

The OCaml manual states that the empty doc-comment is (**). The OCaml lexer claims only that (**) is a empty doc-comment. OCamlformat formats it as (***), which I was surprised to learn has the same AST.

I suggest that the two be unified, either by adding mention of (***) to the OCaml manual or by switching OCamlformat's chosen representation to (**). I'm happy to make the PR to either ocaml/ocaml or OCamlformat depending on which solution we choose.

Additional context

• OCaml syntax highlighting in Tuareg mode (emacs), highlights (**) as a doc-comment and (***) as a regular comment (see https://github.com/ocaml/tuareg/blob/master/tuareg.el#L705).

• The OCaml compiler itself contains occurrences of both (**) and (***).