reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/docs/linux.git/commit/?id=131006cc650ccf5cbd2d6ac1953a263eb25756ea
        git remote add docs https://git.kernel.org/pub/scm/linux/kernel/git/docs/linux.git
        git fetch --no-tags docs docs-next
        git checkout 131006cc650ccf5cbd2d6ac1953a263eb25756ea
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # pip install -r Documentation/sphinx/requirements.txt
        make htmldocs