reproduce:
        # https://github.com/ambarus/linux-0day/commit/26c389f00b6244f87b078f93a9403c8549c53505
        git remote add ambarus https://github.com/ambarus/linux-0day
        git fetch --no-tags ambarus acpm-v4-upstream
        git checkout 26c389f00b6244f87b078f93a9403c8549c53505
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs