reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/sj/linux.git/commit/?id=3695c5fe023f1cd4dae33a4178c619b683b1874f
        git remote add sj https://git.kernel.org/pub/scm/linux/kernel/git/sj/linux.git
        git fetch --no-tags sj damon/next
        git checkout 3695c5fe023f1cd4dae33a4178c619b683b1874f
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs