nixos/qt: improve module documentation

  options = {

    qt = {

      enable = lib.mkEnableOption (lib.mdDoc "Qt theming configuration");

      enable = lib.mkEnableOption "" // {

        description = lib.mdDoc ''

          Whether to enable Qt configuration, including theming.

          Enabling this option is necessary for Qt plugins to work in the

          installed profiles (e.g.: `nix-env -i` or `environment.systemPackages`).

        '';

      };

      platformTheme = lib.mkOption {

        type = with lib.types; nullOr (enum (lib.attrNames platformPackages));