CBQE-0: add bug descr for rebalance_in_with_warming_up

Change-Id: Iba1f5d7ff314e48806e57c733db4f14a9a12398d
Reviewed-on: http://review.couchbase.org/26152
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>