refactor(docs): api: core: add info about assignee object

docs/api/core/ticket-system.md

@@ -78,6 +78,11 @@ They are returned as `List<Ticket>`.
     ts.hasDefaultPagination();
     ```
 
+!!! info "assignees"
+    Tickets have a field of type `Assignee`.
+    These have final String fields,
+    where the system implementation puts values if supported.
+
 !!! example "requesting tickets"
     ```java
     ts.find()