reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=df2218ecdf52896e9cd70687613d016f23bf4919
        git remote add linux-next https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
        git fetch --no-tags linux-next master
        git checkout df2218ecdf52896e9cd70687613d016f23bf4919
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs