reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git/commit/?id=faefb0a59c5914b7b8f737e2ec5c82822e5bc4c7
        git remote add mips-linux https://git.kernel.org/pub/scm/linux/kernel/git/mips/linux.git
        git fetch --no-tags mips-linux mips-next
        git checkout faefb0a59c5914b7b8f737e2ec5c82822e5bc4c7
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs