reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git/commit/?id=8f1cc5242544052e4be037861abc8bc2b89cabda
        git remote add broonie-spi https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git
        git fetch --no-tags broonie-spi for-6.15
        git checkout 8f1cc5242544052e4be037861abc8bc2b89cabda
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs