reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/lee/mfd.git/commit/?id=f4b00ab2c29960961f9641be8f3e8ba4960fd849
        git remote add lee-mfd https://git.kernel.org/pub/scm/linux/kernel/git/lee/mfd.git
        git fetch --no-tags lee-mfd for-mfd-next
        git checkout f4b00ab2c29960961f9641be8f3e8ba4960fd849
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs