DOCSP-34589 clarify multiple mongosync limitations (#271)

source/reference/limitations.txt

@@ -128,13 +128,16 @@ Reversing
   Ensure that unique indexes exist on all shards before reversing.
 - .. include:: /includes/fact-reverse-limitation.rst
 
+.. _multiple-clusters-limitations:
+
 Multiple Clusters
 -----------------
 
-- Syncing multiple source clusters to one destination cluster isn't
-  supported.
-- Syncing one source cluster to many destination clusters isn't
-  supported.
+- ``mongosync`` does not support syncing multiple source clusters to one
+  destination cluster. 
+- One cluster cannot simultaneously be a source cluster in one
+  ``mongosync`` instance and a destination cluster in another
+  ``mongosync`` instance. 
 
 .. _c2c-filtering-limitations:
 

source/reference/mongosync.txt

@@ -316,6 +316,14 @@ a defined sort method, you may need to update those applications to
 specify the expected sort order before the applications work properly
 with the migrated cluster. 
 
+Multiple Clusters
+~~~~~~~~~~~~~~~~~
+
+To sync a source cluster to multiple destination clusters, use one
+``mongosync`` instance for each destination cluster. For more
+information, see :ref:`Multiple Clusters Limitations
+<multiple-clusters-limitations>`. 
+
 .. _c2c-mongosync-examples:
 
 Examples