Documentation/op-guide: highlight defrag operation "--cluster" flag

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

Documentation/op-guide/maintenance.md

@@ -47,7 +47,18 @@ $ etcdctl defrag
 Finished defragmenting etcd member[127.0.0.1:2379]
 ```
 
-Note that defragmentation to a live member blocks the system from reading and writing data while rebuilding its states.
+**Note that defragmentation to a live member blocks the system from reading and writing data while rebuilding its states**.
+
+**Note that defragmentation request does not get replicated over cluster. That is, the request is only applied to the local node. Specify all members in `--endpoints` flag or `--cluster` flag to automatically find all cluster members.**
+
+Run defragment operations for all endpoints in the cluster associated with the default endpoint:
+
+```bash
+$ etcdctl defrag --cluster
+Finished defragmenting etcd member[http://127.0.0.1:2379]
+Finished defragmenting etcd member[http://127.0.0.1:22379]
+Finished defragmenting etcd member[http://127.0.0.1:32379]
+```
 
 To defragment an etcd data directory directly, while etcd is not running, use the command: