Document multi-set operation

Change-Id: Idedcf941681c95db99ddf240daa03c70e5f4a79e
Reviewed-on: http://review.couchbase.org/25334
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>