Rustdoc ommits default for default fn

[bjorn3]

impl<'tcx, T> Value<'tcx> for T {
    default fn from_cycle_error<'a>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> T {
        tcx.sess.abort_if_errors();
        bug!("Value::from_cycle_error called without errors");
    }
}