etcdctl: make doc on 'no-sync' clearer

For issues like https://github.com/coreos/etcd/issues/4496.

etcdctl/README.md

@@ -21,6 +21,8 @@ You can also build etcdctl from source using the build script found in the paren
 
 ### --no-sync
 + don't synchronize cluster information before sending request
++ Use this to access non-published client endpoints
++ Without this flag, values from `--endpoint` flag will be overwritten by etcd cluster when it does internal sync.
 
 ### --output, -o
 + output response in the given format (`simple`, `extended` or `json`)
@@ -40,6 +42,7 @@ You can also build etcdctl from source using the build script found in the paren
 + a comma-delimited list of machine addresses in the cluster
 + default: `"http://127.0.0.1:4001,http://127.0.0.1:2379"`
 + env variable: ETCDCTL_ENDPOINT
++ Without `--no-sync` flag, this will be overwritten by etcd cluster when it does internal sync.
 
 ### --cert-file
 + identify HTTPS client using this SSL certificate file