reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/srini/nvmem.git/commit/?id=1b3f5b88c541318a95fc74db80083b87c1020e03
        git remote add srini-nvmem https://git.kernel.org/pub/scm/linux/kernel/git/srini/nvmem.git
        git fetch --no-tags srini-nvmem for-next
        git checkout 1b3f5b88c541318a95fc74db80083b87c1020e03
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs