reproduce:
        git remote add drm-tip git://anongit.freedesktop.org/drm/drm-tip
        git fetch --no-tags drm-tip drm-tip
        git checkout 723adadf19b6bd8a54881b0e7d04ba56c4e8f401
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs