List enum value in help, optionally.

[smondet]

Like cmdliner, we don't list enum values in the help:

ocaml# type t = { foo: string [@enum ["a", "b"]] [@docv "FOO"];(** Hello *) } [@@deriving cmdliner];;
type t = { foo : string; }
val cmdliner_term : unit -> t Cmdliner.Term.t = <fun>
ocaml# Cmdliner.Term.(eval ~argv:[| "hello"; "--help=plain" |] (cmdliner_term (), info "hello"));;
NAME
        hello
SYNOPSIS
       hello [OPTION]... 

OPTIONS
       --foo=FOO (required)
            Hello 

       --help[=FMT] (default=auto)
           Show this help in format FMT. The value FMT must be one of `auto',
           `pager', `groff' or `plain'. With `auto', the format is `pager` or
           `plain' whenever the TERM env var is `dumb' or undefined.

- : t Cmdliner.Term.result = `Help

It'd be cool to have an option

type t = {
   foo: string [@enum ["a", "b"]] [@docv "FOO"] [@cmdliner.list_enum];
   (** Some help *)
} [@@deriving cmdliner];;