reproduce:
        git remote add lwn git://git.lwn.net/linux.git
        git fetch --no-tags lwn docs-next
        git checkout 991dabb24232d94c04053955af8e85df72a1e2b9
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs