Document mirage Ipaddr conversion

[RyanGibb]

Enabled by mirage/ocaml-ipaddr#117

[patricoferris]

Thanks! This is much cleaner than what we had to do before ^^

[talex5]

I suggest putting it as a section with of_raw, e.g.

  (** {2 Interoperability}

      To convert to or from OCaml Unix addresses, use {!Eio_unix.Ipaddr}.

      To interoperate with the {{:https://opam.ocaml.org/packages/ipaddr/} ipaddr} library:
      - [Ipaddr.to_octets ipaddr_ip |> Eio.Net.Ipaddr.of_raw]
      - [Ipaddr.of_octets_exn (eio_ip :> string)] *)

  val of_raw : string -> v4v6
  (** [of_raw addr] casts [addr] to an IP address.
      @raise Invalid_argument if it is not 4 or 16 bytes long. *)

[RyanGibb]

Thanks for the suggestion @talex5, I've applied it now

[talex5]

Thanks!