Update onprestagechange.md

Based on https://icm.ad.msft.net/imp/v3/incidents/details/162220543/home and the repair item https://dynamicscrm.visualstudio.com/OneCRM/_workitems/edit/1666598 this documentation must be changed until the feature gets fully implemented

Author: hedelg

powerapps-docs/developer/model-driven-apps/clientapi/reference/events/onprestagechange.md

@@ -33,6 +33,8 @@ An execution context object is passed to event handlers for this event. You can
 
 This client API is only supported on the unified client. The legacy web client does not support this client API.
 
+onPreStageChange is only supported for single entity Business Process Flows. It is not supported for cross-entity processes.
+
 ## Methods supported for this event
 - **formContext.data.process**.[addOnPreStageChange](../formcontext-data-process/eventhandlers/addOnPreStageChange.md) method to add event handlers for this event.
 - **formContext.data.process**.[removeOnPreStageChange](../formcontext-data-process/eventhandlers/removeOnPreStageChange.md) method to remove event handlers for this event.