nixos/roundcube: add example for `database.passwordFile` (#348166)

        default = "";

      };

      passwordFile = lib.mkOption {

        type = lib.types.str;

        type = lib.types.path;

        example = lib.literalExpression ''

          pkgs.writeText "roundcube-postgres-passwd.txt" '''

            hostname:port:database:username:password

          '''

        '';

        description = ''

          Password file for the postgresql connection.

          Must be formatted according to PostgreSQL .pgpass standard (see https://www.postgresql.org/docs/current/libpq-pgpass.html)