DOCSP-48515: Source/destination balancing (mongodb#678)

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(cherry picked from commit c1b61ee)

Author: mayaraman19

source/reference/mongosync/mongosync-behavior.txt

@@ -114,6 +114,17 @@ destination cluster.
 Chunk Distribution
 ''''''''''''''''''
 
+.. important::
+   
+   Even if the source cluster is balanced, ``mongosync`` doesn't
+   ensure balance of the destination cluster. Because ``mongosync``
+   doesn't support the execution of sharding operations during
+   migration, you must wait until it is safe to accept writes 
+   to rebalance the destination cluster. See :ref:`sharding-balancing`
+   for guidance on how to rebalance the cluster and 
+   :ref:`sharded cluster limitations <c2c-sharded-limitations>`
+   for information on sharded cluster limitations in ``mongosync``.
+
 ``mongosync`` does not preserve chunk distribution from the source to
 the destination, even with multiple ``mongosync`` instances. It is not
 possible to reproduce a particular pre-split of chunks from a source