reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/frank.li/linux.git/commit/?id=3b4531c6e0f4c8874f0266853a410438eda1fc24
        git remote add frank-li https://git.kernel.org/pub/scm/linux/kernel/git/frank.li/linux.git
        git fetch --no-tags frank-li for-next
        git checkout 3b4531c6e0f4c8874f0266853a410438eda1fc24
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs