[linux-stable-rc:linux-4.9.y 9966/9999] htmldocs: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.

[kernel test robot]

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git linux-4.9.y
head:   6940a98776c9014fdcc806e39ab0ebc8316cf241
commit: 2808035ba55eb8aaaf5eb37421dbfff37c1f25a8 [9966/9999] x86/speculation: Add SRBDS vulnerability and mitigation documentation
reproduce: make htmldocs

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@xxxxxxxxx>

All warnings (new ones prefixed by >>):

warning: jobserver unavailable: using -j1. Add '+' to parent make rule.

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0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@xxxxxxxxxxxx

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