reproduce:
        git remote add lwn git://git.lwn.net/linux.git
        git fetch --no-tags lwn docs-next
        git checkout e6a5af90f0a24f08445e3b8a11b727ac84a9520c
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs