DOCSP-39095 adding atlas search indexes limitation (mongodb#312)

* DOCSP-39095 adding atlas search indexes limitation

* DOCSP-39095 tech edit

* DOCSP-39095 tech edit

* DOCSP-39095 tech edit

Author: ltran-mdb2

source/reference/limitations.txt

@@ -54,6 +54,8 @@ General Limitations
 - .. include:: /includes/fact-applyOps.rst
 - ``mongosync`` must read from the source cluster using the 
   :readmode:`primary` read preference.
+- ``mongosync`` does not support syncing 
+  `Atlas Search Indexes <https://www.mongodb.com/docs/atlas/atlas-search/create-index/>`__.
 
 MongoDB Community Edition
 -------------------------