nixos/syncthing: point out pitfalls with extraOptions (#210208)

        description = mdDoc ''

          Extra configuration options for Syncthing.

          See <https://docs.syncthing.net/users/config.html>.

          Note that this attribute set does not exactly match the documented

          xml format. Instead, this is the format of the json rest api. There

          are slight differences. For example, this xml:

          ```xml

          <options>

            <listenAddress>default</listenAddress>

            <minHomeDiskFree unit="%">1</minHomeDiskFree>

          </options>

          ```

          corresponds to the json:

          ```json

          {

            options: {

              listenAddresses = [

                "default"

              ];

              minHomeDiskFree = {

                unit = "%";

                value = 1;

              };

            };

          }

          ```

        '';

        example = {

          options.localAnnounceEnabled = false;