clarified use of external store

Author: Nicolas Oman

articles/service-fabric/service-fabric-common-questions.md

@@ -143,7 +143,8 @@ Reliable collections are typically [partitioned](service-fabric-concepts-partiti
 
 - Create a service that queries all partitions of another service to pull in the required data.
 - Create a service that can receive data from all partitions of another service.
-- Periodically push data from each service to an external store. This approach is only appropriate if the queries you're performing are not part of your core business logic.
+- Periodically push data from each service to an external store. This approach is only appropriate if the queries you're performing are not part of your core business logic, as the external store's data will be stale.
+- Alternatively, store data that must support querying across all records directly in a data store rather than in a reliable collection. This eliminates the issue with stale data, but doesn't allow the advantages of reliable collections to be leveraged.
 
 
 ### What's the best way to query data across my actors?