add explaination

Author: shuangela

source/reference/api/start.txt

@@ -365,6 +365,10 @@ Response:
 Start Sync from Replica Set to Sharded Cluster
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+The following example starts a sync job between the replica set ``cluster0`` and the
+sharded cluster ``cluster1``. The source cluster is ``cluster0`` and the destination
+cluster is ``cluster1``. The ``key`` array in this example defines the shard key ``{"location": 1, "region": 1 }``.
+
 Request:
 
 .. literalinclude:: /includes/api/requests/start-rs-shard.sh