releasenotes.txt: Breaking change for IPersistentCollection.GetSnapshot().

Author: oskarb

releasenotes.txt

@@ -6,6 +6,7 @@ Build vNext
     ##### Possible Breaking Changes #####
         * IDeleteEventListener, IEventSource: Use generic ISet<> instead of non-generic in method signatures.
         * SqlString.Parts removed. Use SqlString.Count and SqlString.GetEnumerator().
+        * IPersistentCollection.GetSnapshot() now returns object instead of ICollection. The snapshot should be opaque to outside code.
 
 Build 3.3.1.GA
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