Document /controller/rebalance as of pre delta recovery changes.

Change-Id: Iad68ce39e5951caba66b680b4945e75282dd3766
Reviewed-on: http://review.couchbase.org/34274
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>