sched: Update comments for CLONE_NEWNS

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-api@vger.kernel.org
Link: http://lkml.kernel.org/r/1412674147-8941-1-git-send-email-chenhanxiao@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>