DOCSP-48400-confusion-multi-shard-key (mongodb#732)

* add explaination

* condense wording

* add source and destination

(cherry picked from commit 9734cd3)

Author: shuangela

source/includes/intro-start-api-example-intro.rst

@@ -1,3 +1,2 @@
-The following example starts a sync job between ``cluster0`` and
-``cluster1``.  The source cluster is ``cluster0`` and the destination
-cluster is ``cluster1``.
+The following example starts a sync job between the source cluster ``cluster0`` and the
+destination cluster ``cluster1``.

source/reference/api/start.txt

@@ -360,6 +360,10 @@ Response:
 Start Sync from Replica Set to Sharded Cluster
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+The following example starts a sync job between the source replica set ``cluster0`` and the
+destination sharded cluster ``cluster1``. The ``key`` array in this example defines the shard key
+``{"location": 1, "region": 1 }``.
+
 Request:
 
 .. literalinclude:: /includes/api/requests/start-rs-shard.sh