reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git/commit/?id=357dfc0976c9d5cd147b50d1d44d98ea75e3ff38
        git remote add robh https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git
        git fetch --no-tags robh dt-convert-irq
        git checkout 357dfc0976c9d5cd147b50d1d44d98ea75e3ff38
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs