Update 3.13.0 release notes

Author: ansd

release-notes/3.13.0.md

@@ -87,7 +87,9 @@ GET /api/queues` HTTP API endpoint has dropped several rarely used metrics, resu
 
 ### MQTT Plugin
 
-`mqtt.subscription_ttl` configuration setting was replaced with `mqtt.max_session_expiry_interval_seconds`.
+`mqtt.subscription_ttl` (in milliseconds) configuration setting was replaced with `mqtt.max_session_expiry_interval_seconds` (in seconds).
+A 3.13 RabbitMQ node will fail to boot if the old configuration setting is set.
+For example, if you set `mqtt.subscription_ttl = 3600000` (1 hour) prior to 3.13, replace that setting with `mqtt.max_session_expiry_interval_seconds = 3600` (1 hour) in 3.13.
 
 
 ### Getting Help