DOCSP-37061-mongosync-timestamp-limitation (#380)

* DOCSP-37061-mongosync-timestamp-limitation

* *

* reword to match cloud docs copy

Author: ajhuh-mdb

source/reference/limitations.txt

@@ -61,6 +61,8 @@ General Limitations
   <https://www.mongodb.com/docs/atlas/atlas-search/create-index/>`__.
 - ``mongosync`` only supports clusters that use the :ref:`WiredTiger 
   <storage-wiredtiger>` storage engine.
+- You can't sync a collection with any documents that have an empty timestamp,
+  such as ``Timestamp(0,0)``. 
 
 MongoDB Community Edition
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