Add CLUSTER INFO fields (redis#1798)

Author: enjoy-binbin

commands/cluster-info.md

@@ -1,6 +1,5 @@
-`CLUSTER INFO` provides `INFO` style information about Redis Cluster
-vital parameters. The following is a sample output, followed by the
-description of each field reported.
+`CLUSTER INFO` provides `INFO` style information about Redis Cluster vital parameters.
+The following fields are always present in the reply:
 
 ```
 cluster_state:ok
@@ -30,7 +29,23 @@ total_cluster_links_buffer_limit_exceeded:0
 * `cluster_stats_messages_received`: Number of messages received via the cluster node-to-node binary bus.
 * `total_cluster_links_buffer_limit_exceeded`: Accumulated count of cluster links freed due to exceeding the `cluster-link-sendbuf-limit` configuration.
 
-More information about the Current Epoch and Config Epoch variables are available in the Redis Cluster specification document.
+The following message-related fields may be included in the reply if the value is not 0:
+Each message type includes statistics on the number of messages sent and received.
+Here are the explanation of these fields:
+
+* `cluster_stats_messages_ping_sent` and `cluster_stats_messages_ping_received`: Cluster bus PING (not to be confused with the client command `PING`).
+* `cluster_stats_messages_pong_sent` and `cluster_stats_messages_pong_received`: PONG (reply to PING).
+* `cluster_stats_messages_meet_sent` and `cluster_stats_messages_meet_received`: Handshake message sent to a new node, either through gossip or `CLUSTER MEET`.
+* `cluster_stats_messages_fail_sent` and `cluster_stats_messages_fail_received`: Mark node xxx as failing.
+* `cluster_stats_messages_publish_sent` and `cluster_stats_messages_publish_received`: Pub/Sub Publish propagation, see [Pubsub](/topics/pubsub#pubsub).
+* `cluster_stats_messages_auth-req_sent` and `cluster_stats_messages_auth-req_received`: Replica initiated leader election to replace its master.
+* `cluster_stats_messages_auth-ack_sent` and `cluster_stats_messages_auth-ack_received`: Message indicating a vote during leader election.
+* `cluster_stats_messages_update_sent` and `cluster_stats_messages_update_received`: Another node slots configuration.
+* `cluster_stats_messages_mfstart_sent` and `cluster_stats_messages_mfstart_received`: Pause clients for manual failover.
+* `cluster_stats_messages_module_sent` and `cluster_stats_messages_module_received`: Module cluster API message.
+* `cluster_stats_messages_publishshard_sent` and `cluster_stats_messages_publishshard_received`: Pub/Sub Publish shard propagation, see [Sharded Pubsub](/topics/pubsub#sharded-pubsub).
+
+More information about the Current Epoch and Config Epoch variables are available in the [Redis Cluster specification document](/topics/cluster-spec#cluster-current-epoch).
 
 @return