Add WS related info on what is sent in case of :session_killed

CHANGELOG.md

@@ -10,7 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - Added longpolling as new connection type [#217](https://github.com/Accenture/reactive-interaction-gateway/issues/217)
-- When terminating an SSE connection after its associated session has been blacklisted, RIG now sends out a `rig.session_killed` event before closing the socket.
+- When terminating an SSE connection after its associated session has been blacklisted, RIG now sends out a `rig.session_killed` event before closing the socket. For WebSocket connections, the closing frame contains "Session killed." as its payload.
 - New API for querying and updating the session blacklist: `/v2/session-blacklist`, which introduces the following breaking changes:
   - When a session has been added to the session blacklist successfully, the endpoint now uses the correct HTTP status code "201 Created" instead of "200 Ok".
   - When using the API to blacklist a session, the `validityInSeconds` should now be passed as an integer value (using a string still works though).