Fix unexpected-docstring warning when building with dune

[kit-ty-kate]

$ dune build
File "src/cmdliner.mli", line 577, characters 4-57:
577 |     (** The type for environment variable information. *)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error (warning 50 [unexpected-docstring]): ambiguous documentation comment
File "src/cmdliner.mli", lines 803-806, characters 2-78:
803 | ..(** The type for argument converters.
804 | 
805 |       {b Warning.} Do not use directly, use {!val-conv} or {!val-conv'}.
806 |       This type will become abstract in the next major version of cmdliner. *)
Error (warning 50 [unexpected-docstring]): ambiguous documentation comment

Tested with OCaml 4.14.0

[dbuenzli]

Sorry but that's not a good fix.

Could you please rather make the dune set of warnings coincide with the default one.

Otherwise we'll keep ping-ponging on these issues as I'm not using the dune build (or even better convince dune people to stop making their own set of user hostile warning defaults).

[kit-ty-kate]

Done: https://github.com/dbuenzli/cmdliner/pull/171