Doc cautions about Redis Cluster keyspace events.

Due to #1111, applications can suffer from missing important Keyspace events as described at spring-projects/spring-session#2230.

This adds a caution to the docs to help others avoid running into this problem.

Author: kinsersh

Diff for: src/main/antora/modules/ROOT/pages/redis/cluster.adoc

@@ -6,6 +6,9 @@ See the https://redis.io/topics/cluster-tutorial[Cluster Tutorial] for more info
 
 NOTE: When using xref:repositories.adoc[Redis Repositories] with Redis Cluster, make yourself familiar with how to xref:redis/redis-repositories/cluster.adoc[run Redis Repositories on a Cluster].
 
+CAUTION: Relying on keyspace events when using Redis Cluster is discouraged due to https://github.com/spring-projects/spring-data-redis/issues/1111[only being subscribed to events from one random redis node in the cluster].
+Using redis without multiple shards avoids this problem.
+
 [[cluster.working.with.cluster]]
 == Working With Redis Cluster Connection