reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=243996d172a68fba79765cfe20ab910c29ec5f50
        git remote add netdev-net-next https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git
        git fetch --no-tags netdev-net-next main
        git checkout 243996d172a68fba79765cfe20ab910c29ec5f50
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs