reproduce:
        # https://github.com/steev/linux/commit/70ef2cc4d51e006d23e5d2559f395e1c1e4e7dbc
        git remote add steev https://github.com/steev/linux
        git fetch --no-tags steev linux-5.10-gen-rkr3.4
        git checkout 70ef2cc4d51e006d23e5d2559f395e1c1e4e7dbc
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs