Documentation/op-guide: Clarify TLS "--peer" flags

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

Documentation/op-guide/configuration.md

@@ -271,12 +271,12 @@ The security flags help to [build a secure etcd cluster][security].
 + env variable: ETCD_PEER_CA_FILE
 
 ### --peer-cert-file
-+ Path to the peer server TLS cert file.
++ Path to the peer server TLS cert file. This is the cert for peer-to-peer traffic, used both for server and client.
 + default: ""
 + env variable: ETCD_PEER_CERT_FILE
 
 ### --peer-key-file
-+ Path to the peer server TLS key file.
++ Path to the peer server TLS key file. This is the key for peer-to-peer traffic, used both for server and client.
 + default: ""
 + env variable: ETCD_PEER_KEY_FILE