reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=1f455f601e2060497f9883991e8d5e79fbc7b047
        git remote add tip https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
        git fetch --no-tags tip timers/core
        git checkout 1f455f601e2060497f9883991e8d5e79fbc7b047
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs