reproduce:
        git remote add lwn git://git.lwn.net/linux.git
        git fetch lwn docs-next
        git checkout lwn/docs-next
        b4 shazam https://lore.kernel.org/r/87y12m1zk4.fsf@trenco.lwn.net
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs