OP: add annotation re. cluster upgrade and RQE (#1555)

Author: dwdougherty

content/operate/oss_and_stack/install/upgrade/cluster.md

@@ -15,7 +15,7 @@ The supported upgrade paths are:
 - Redis Open Source 7.x (with or without modules) to Redis 8 in Redis Open Source
 - Redis Stack 7.2 or 7.4 to Redis 8 in Redis Open Source
 {{< /note >}}
-
+</br>
 {{< note >}}
 It's essential to practice upgrading Redis in a controlled environment before upgrading it in a production environment.
 Docker is an excellent tool to use for this purpose.
@@ -55,6 +55,11 @@ Upgrade each node one at a time, starting with the replicas, using these steps:
 
 If necessary, restore the saved files to their original locations on each node. Then restart Redis on each node.
 
+{{< warning >}}
+Redis Query Engine won't be fully operational until all master shards have completed their upgrades.
+Any queries sent during the upgrade process will either return errors or a message such as "Waiting for all nodes to connect".
+{{< /warning >}}
+
 ### Verify the upgrade
 
 ```bash