rcu: Kconfig help needs to say that TREE_PREEMPT_RCU scales down

To quote Valdis:

    This leaves somebody who has a laptop wondering which
    choice is best for a system with only one or two cores that
    has CONFIG_PREEMPT defined. One choice says it scales down
    nicely, the other explicitly has a 'depends on PREEMPT'
    attached to it...

So add "scales down nicely" to TREE_PREEMPT_RCU to match that of
TREE_RCU.

Suggested-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
LKML-Reference: <12528585112362-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>