added rebalance documentation

Change-Id: I4981d25a1d025293345291fab7edab56250b6cdd
Reviewed-on: http://review.couchbase.org/28823
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>