reproduce:
        git remote add lwn git://git.lwn.net/linux.git
        git fetch --no-tags lwn docs-next
        git checkout f5c7cc77acf5abebc5aec4f4236954c23f29e09b
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs