sched/Documentation: Update sched-design-CFS.txt documentation

sched/Documentation: Update sched-design-CFS.txt documentation

2bd2d6f2dc ("sched: Replace use of entity_key()") had remove the
entity_key() which function is to subtract a tasks vruntime by
its groups minvruntime as the rbtree key.
The phrase "there is a subtraction using rq->cfs.min_vruntime
to account for possible wraparounds" in documentation feels strange
and meaningless. So remove it.

Signed-off-by: Li Bin <huawei.libin@huawei.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1378706740-7500-1-git-send-email-huawei.libin@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>