add additional info

Author: amalhotra-mdb

source/reference/beta-program-private/many-to-one.txt

@@ -49,6 +49,13 @@ the following parameter to the :ref:`/start <c2c-api-start>` requests:
 
    "destinationDataHandling": "ignorePreExistingNamespaces"
 
+Internally, ``mongosync`` uses the ``--migrationName`` command line option 
+to create a customized migration metadata database for each migration 
+following the ``mongosync_internal_<migrationName>`` pattern. Setting 
+``"destinationDataHandling"`` to ``"ignorePreExistingNamespaces"`` allows
+each migration to write to the destination cluster that may have 
+pre-existing namespaces from the other many-to-one migrations.
+
 Behavior 
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