message: Document ParameterKind::Optional
diff: @@ -66,6 +66,10 @@ pub(crate) enum FunctionError {
  #[repr(C)]
  pub(crate) enum ParameterKind<V> {
  Required,
  + /// When optional parameter is not supplied, there's no error,
  + /// but the slot remains `None`.
  + ///
  + /// This is used only in native code, parameters of type `Option<T>` become `Optional`.
  Optional,
  Defaulted(V),
  Args,
  