Docs: Add docs for Redis cluster caching backend configuration (#39687)

* caching: add docs for redis cluster config

* address PR comments

Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>

Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>

docs/sources/enterprise/enterprise-configuration.md

@@ -401,6 +401,12 @@ The full Redis URL of your Redis server. Example: `redis://localhost:6739/0`.
 
 The default is `"redis://localhost:6379"`.
 
+### cluster
+
+A comma-separated list of Redis cluster members in `host:port` format. For example, `localhost:7000, localhost: 7001, localhost:7002`.
+
+> **Note:** If you have specify `cluster`, the value for `url` is ignored.
+
 ### prefix
 
 A string that prefixes all Redis keys. This value must be set if using a shared database in Redis. If `prefix` is empty, then one will not be used.