Update 4.0 release notes

References #11937.

(cherry picked from commit e7296c1)

Author: michaelklishin

release-notes/4.0.0.md

@@ -1,6 +1,6 @@
-## RabbitMQ 4.0.0-beta.6
+## RabbitMQ 4.0.0-beta.7
 
-RabbitMQ `4.0.0-beta.6` is a preview of a new major release.
+RabbitMQ `4.0.0-beta.7` is a preview of a new major release.
 
 Starting June 1st, 2024, community support for this series will only be provided to [regularly contributing users](https://github.com/rabbitmq/rabbitmq-server/blob/main/COMMUNITY_SUPPORT.md) and those who hold a valid [commercial support license](https://tanzu.vmware.com/rabbitmq/oss).
 
@@ -45,6 +45,14 @@ on how to migrate to quorum queues for the parts of the system that really need
 ### Quorum Queues Now Have a Default Redelivery Limit
 
 Quorum queues now have a default [redelivery limit](https://www.rabbitmq.com/docs/next/quorum-queues#poison-message-handling) set to `20`.
+Messages that are redelivered 20 times or more will be [dead-lettered](https://www.rabbitmq.com/docs/dlx) or dropped (removed).
+
+This limit is necessary to protect nodes from consumers that run into infinite crash-requeue-crash-requeue loops. Such
+consumers can drive a node out of disk space by making a quorum queue Raft log grow forever without allowing compaction
+of older entries.
+
+If 20 deliveries per message is a common scenario for a queue, a dead-lettering target must be configured
+for such queues. The recommended way of doing that is via a [policy](https://www.rabbitmq.com/docs/parameters#policies).
 
 ### CQv1 Storage Implementation was Removed
 
@@ -93,7 +101,8 @@ IDs are passed around in messages instead of the entire payload.
 
 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).
+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).
 
 ### MQTT
 
@@ -158,11 +167,13 @@ periods of time (no more than a few hours).
 
 ### Recommended Post-upgrade Procedures
 
-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.
+#### Configure Dead Lettering for Frequently Redelivered Messages
+
+In environments where messages can experience 20 redeliveries, the affected queues should have [dead lettering](https://www.rabbitmq.com/docs/dlx)
+configured (usually via a [policy](https://www.rabbitmq.com/docs/parameters#policies)) to make sure
+that messages that are redelivered 20 times are moved to a separate queue (or stream) instead of
+being dropped (removed) by the [crash-requeue-redelivery loop protection mechanism](https://www.rabbitmq.com/docs/next/quorum-queues#poison-message-handling).
 
-TBD
 
 ## Changes Worth Mentioning