Add docs about default values in config

[kpp]

    settings.set_default("log-type", "Stderr").expect("Can't set default value for `log-type`");
    settings.set_default("motd", "This is tox-rs").expect("Can't set default value for `motd`");
    settings.set_default("lan-discovery", "False").expect("Can't set default value for `lan-discovery`");
    settings.set_default("threads", "1").expect("Can't set default value for `threads`");
    settings.set_default("tcp-connections-limit", "512").expect("Can't set default value for `tcp-connections-limit`");

This should be documented explicitly that these values are equal to default values from args.