reproduce:
        git checkout 44badc908f2c85711cb18e45e13119c10ad3a05f
        b4 shazam https://lore.kernel.org/r/20241002-b4-ovpn-v8-3-37ceffcffbde@openvpn.net
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs