doc: Eliminate cond_resched_rcu_qs() in favor of cond_resched()

Now that cond_resched() also provides RCU quiescent states when
needed, it can be used in place of cond_resched_rcu_qs().  This
commit therefore documents this change.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>