reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/broonie/ci.git/commit/?id=82116e539ffb7ce0c317c208d53d2126cdcee687
        git remote add broonie-ci https://git.kernel.org/pub/scm/linux/kernel/git/broonie/ci.git
        git fetch --no-tags broonie-ci fileY2I0bM
        git checkout 82116e539ffb7ce0c317c208d53d2126cdcee687
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs