reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=53ed3233e6b5fbfb63abc945d1fc156222f4eb62
        git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
        git fetch --no-tags linus master
        git checkout 53ed3233e6b5fbfb63abc945d1fc156222f4eb62
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs