reproduce:
        git remote add ti git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git
        git fetch --no-tags ti ti-linux-6.1.y-wip
        git checkout 1d2f40c03c3b1d45809a245d79913b28c613f297
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs