reproduce:
        # https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?id=38bcd10de11e3ae4b49e93a5bd12a490aafc6fed
        git remote add akpm-mm https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git
        git fetch --no-tags akpm-mm master
        git checkout 38bcd10de11e3ae4b49e93a5bd12a490aafc6fed
        make menuconfig
        # enable CONFIG_COMPILE_TEST, CONFIG_WARN_MISSING_DOCUMENTS, CONFIG_WARN_ABI_ERRORS
        # install Sphinx 2.4.4
        make htmldocs