Update release notes with details of the delivery-limit default.

(cherry picked from commit 1cf530a)

Author: kjnilsson

release-notes/4.0.0.md

@@ -33,6 +33,7 @@ See Compatibility Notes below to learn about **breaking or potentially breaking
 * RabbitMQ 3.13 `rabbitmq.conf` setting `rabbitmq_amqp1_0.default_vhost` is unsupported in RabbitMQ 4.0.
   Instead `default_vhost` will be used to determine the default vhost an AMQP 1.0 client connects to(i.e. when the AMQP 1.0 client does not define the vhost in the `hostname` field of the `open` frame)
 * RabbitMQ Shovels will be able connect to a RabbitMQ 4.0 node via AMQP 1.0 only when the Shovel runs on a RabbitMQ node >= `3.13.7`
+* Quorum queues will now always set a default `delivery-limit` of 20 which can be increased or decreased by policies and queue arguments but cannot be unset. Some applications or configurations may need to be updated to handle this.
 
 ## Erlang/OTP Compatibility Notes
 
@@ -83,8 +84,11 @@ periods of time (no more than a few hours).
 
 ### Recommended Post-upgrade Procedures
 
-TBD
+Set a low priority dead lettering policy for all quorum queues to dead letter to a stream or similar
+so that messages that reach the new default delivery limit of 20 aren't lost completely
+when no dead lettering policy is in place.
 
+TBD
 
 ## Changes Worth Mentioning