reproduce:
        git remote add chrome-os https://chromium.googlesource.com/chromiumos/third_party/kernel
        git fetch --no-tags chrome-os chromeos-6.6
        git checkout 3347e43f1763e946c8db116bb3241731d1417cab
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs