Document instance migration phases 3 and 4

docs/apis-tools/zeebe-api/gateway-service.md

@@ -861,6 +861,9 @@ Returned if:
 - A mapping instruction refers to an unsupported element (i.e. some elements will be supported later on)
 - A mapping instruction refers to element in unsupported scenarios.
  (i.e. migrating active elements with event subscriptions will be supported later on)
+- A mapping instruction detaches a boundary event from an active element
+- Multiple mapping instructions refer to the same catch event
+- A mapping instruction changes a parallel multi-instance body to a sequential multi-instance body or vice versa
 
 ## `PublishMessage` RPC