nixos/glance: clarify settings _secret

        for more.

        Settings containing secret data should be set to an

        attribute set containing the attribute

        <literal>_secret</literal> - a string pointing to a file

        containing the value the option should be set to. See the

        example in `services.glance.settings.pages` at the weather widget

        attribute set with this format: `{ _secret = "/path/to/secret"; }`.

        See the example in `services.glance.settings.pages` at the weather widget

        with a location secret to get a better picture of this.

      '';

    };