4.0.1 release notes: clarify that Blue/Green deployments are an option for 3.13.x clusters with Khepri enabled

Author: michaelklishin

release-notes/4.0.1.md

@@ -195,7 +195,9 @@ for release notes of individual releases.
 This release series only supports upgrades from `3.13.x`.
 
 This release requires **all feature flags** in the 3.x series (specifically `3.13.x`) to be enabled before upgrading,
-there is no upgrade path from 3.12.14 (or a later patch release) straight to `4.0.0`.
+there is no direct upgrade path from 3.12.14 (or a later patch release) straight to a `4.0.x` version.
+
+[Blue/Green Deployment](https://www.rabbitmq.com/docs/blue-green-upgrade)-style upgrades are avaialble for migrations from 3.12.14 to `4.0.x`.
 
 ### Required Feature Flags
 
@@ -204,6 +206,13 @@ This release [graduates](https://www.rabbitmq.com/docs/feature-flags#graduation)
 All users must enable all stable [feature flags] before upgrading to 4.0 from
 the latest available 3.13.x patch release.
 
+### Migrating 3.13.x Clusters with Khepri Enabled
+
+Khepri was an experimental feature in the `3.13.x` series. There is no direct upgrade path for clusters on `3.13.x` with Khepri enabled to `4.0.x`,
+because internal data model used to store various metadata (users, virtual hosts, queues, streams, policies, and so on) has changed dramatically.
+
+Such clusters should be [migrated using the Blue/Green deployment strategy](https://www.rabbitmq.com/docs/blue-green-upgrade).
+
 ### Mixed version cluster compatibility
 
 RabbitMQ 4.0.0 nodes can run alongside `3.13.x` nodes. `4.0.x`-specific features can only be made available when all nodes in the cluster