reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/commit/?id=bca6f242396960d86ae46f11349cd87caede3c62
        git remote add cel https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux
        git fetch --no-tags cel lkxdrgen
        git checkout bca6f242396960d86ae46f11349cd87caede3c62
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs