change warning (mongodb#733)

Author: shuangela

source/reference/api/start.txt

@@ -102,8 +102,8 @@ Request Body Parameters
 
          .. include:: /includes/fact-verifier-buildIndexes
 
-         :red:`WARNING:` Do **not** manually build indexes while ``mongosync`` is 
-         performing a migration.  Wait until the migration is fully 
+         :red:`WARNING:` Do **not** manually build indexes on the destination cluster
+         while ``mongosync`` is performing a migration.  Wait until the migration is fully
          committed.
 
          For more information on the indexes it does build, see