etcdctl: highlight "defrag" command caveats

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

etcdctl/README.md

@@ -878,6 +878,10 @@ If NOSPACE alarm is present:
 
 DEFRAG defragments the backend database file for a set of given endpoints while etcd is running, or directly defragments an etcd data directory while etcd is not running. When an etcd member reclaims storage space from deleted and compacted keys, the space is kept in a free list and the database file remains the same size. By defragmenting the database, the etcd member releases this free space back to the file system.
 
+**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.**
+
 #### Options
 
 - data-dir -- Optional. If present, defragments a data directory not in use by etcd.