3.12.0 release notes: a warning on those upgrading to 3.12 and Erlang 26 at the same time

Author: michaelklishin

release-notes/3.12.0.md

@@ -59,9 +59,19 @@ similarly to how [3.11.0 required all feature flags introduced before 3.9.0](htt
 
 If the feature flags are not enabled, RabbitMQ 3.12 and later nodes will refuse to start.
 
+### Upgrading to Erlang 26 and RabbitMQ 3.12 at the Same Time
+
+When upgrading from 3.11.x on Erlang 25 to 3.12.x on Erlang 26,
+the cluster must be on **at least version 3.11.17** for a [safe upgrade](https://github.com/rabbitmq/ra/pull/369) of quorum queues.
+
+Alternatively, it is possible to upgrade all nodes to 3.12.0 on Erlang 25 from an earlier
+3.11.x version, then upgrade the 3.12 cluster to Erlang 26.
+
 ### Mixed version cluster compatibility
 
-RabbitMQ 3.12.0 nodes can run alongside `3.11.x` nodes. `3.12.x`-specific features can only be made available when all nodes in the cluster
+RabbitMQ 3.12.0 nodes can run alongside `3.11.x` nodes starting with `3.11.17`.
+
+`3.12.x`-specific features can only be made available when all nodes in the cluster
 upgrade to 3.12.0 or any other patch release in the new series.
 
 While operating in mixed version mode, some aspects of the system may not behave as expected. The list of known behavior changes is covered below.
@@ -71,7 +81,6 @@ Mixed version clusters are a mechanism that allows rolling upgrade and are not m
 periods of time (no more than a few hours).
 
 
-
 ## Compatibility Notes
 
 ### More Feature Flags Gratuate to Core Features ("Always Enabled")