Explain how to remove failed cluster node (redis#2427) (redis#2434)

`redis-cli --cluster del-node` doesn't work since it tries to connect to all nodes. `--cluster call CLUSTER FORGET` works though.

Author: jiangyunpeng

docs/management/scaling.md

@@ -875,6 +875,14 @@ over one of its replicas and remove the node after it turned into a replica of t
 new master. Obviously this does not help when you want to reduce the actual
 number of masters in your cluster, in that case, a resharding is needed.
 
+There is a special scenario where you want to remove a failed node.
+You should not use the `del-node` command because it tries to connect to all nodes and you will encounter a "connection refused" error.
+Instead, you can use the `call` command:
+
+ redis-cli --cluster call 127.0.0.1:7000 cluster forget `<node-id>`
+
+This command will execute `CLUSTER FORGET` command on every node. 
+
 #### Replica migration
 
 In Redis Cluster, you can reconfigure a replica to replicate with a