nixos/nixpkgs: Rewrite overlays option docs

        '';

      type = types.listOf overlayType;

      description = lib.mdDoc ''

        List of overlays to use with the Nix Packages collection.

        (For details, see the Nixpkgs documentation.)  It allows

        you to override packages globally. Each function in the list

        takes as an argument the *original* Nixpkgs.

        The first argument should be used for finding dependencies, and

        the second should be used for overriding recipes.

        If `nixpkgs.pkgs` is set, overlays specified here

        will be applied after the overlays that were already present

        in `nixpkgs.pkgs`.

        List of overlays to apply to Nixpkgs.

        This option allows modifying the Nixpkgs package set accessed through the `pkgs` module argument.

        For details, see the [Overlays chapter in the Nixpkgs manual](https://nixos.org/manual/nixpkgs/stable/#chap-overlays).

        If the {option}`nixpkgs.pkgs` option is set, overlays specified using `nixpkgs.overlays` will be applied after the overlays that were already included in `nixpkgs.pkgs`.

      '';

    };