nixos/matrix-conduit: improve documentation

      default = null;

      example = "/run/secrets/matrix-conduit.env";

      description = ''

        Path to file containing sensitive environment variables.

        Path to a file containing sensitive environment as described in {manpage}`systemd.exec(5).

        Some variables that can be considered secrets are:

        - CONDUIT_JWT_SECRET:

          The secret used in the JWT to enable JWT login without it a 400 error will be returned

          The secret used to enable JWT login. Without it a 400 error will be returned.

        - CONDUIT_TURN_SECRET:

          The TURN secret

        <https://docs.conduit.rs/configuration.html>

        for details on supported values.

        Note that database_path can not be edited because the service's reliance on systemd StateDir.

        For secrets use secretFile option instead.

        For secrets use the `secretFile` option instead.

      '';

    };

  };