reproduce:
        git remote add sailus-media-tree git://linuxtv.org/sailus/media_tree.git
        git fetch --no-tags sailus-media-tree devel
        git checkout bcf999e34bc7c56a4cc589a2aadfcf248bbc3f1c
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs