reproduce:
        git checkout 4cece764965020c22cff7665b18a012006359095
        b4 shazam https://lore.kernel.org/r/171291193308.3532867.129739584130889725.stgit@dwillia2-xfh.jf.intel.com
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs