You're right that narrowing isn't exactly a minor mode. However, standard-mode-line-format includes narrowing information as part of mode-line-modes. So the narrowing indicator is rendered inside the same parentheses as indicators for true major and minor modes. Since describe-mode provides documentation for everything else that's inside those parentheses, it seemed appropriate to document narrowing in the same place.
When I first started using emacs some years ago, I remember being confused why "Narrow" was the only thing in the mode list that wasn't mentioned by describe-mode. I understand it now, but I think others might face similar confusion with the current behavior. I think this patch would address that.