In the course of investigating #3781, I noticed that [`addnodes.literal_emphasis` and `addnodes.literal_strong`](https://github.com/sphinx-doc/sphinx/blob/8f9dd6499207be2f3291a0982ed02e418730ef5e/sphinx/addnodes.py#L269-L278) do not subclass from `docutils.nodes.literal`. Thus, AFAICT they do not render as literal text. Is this intended behavior?