etcdserver: improve discovery ignore warning

Documentation/0.5/clustering.md

@@ -199,7 +199,7 @@ ignored on this machine.
 ```
 $ etcd -name infra0 -initial-advertise-peer-urls http://10.0.1.10:2380 \
   -discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de
-etcdserver: warn: ignoring discovery: etcd has already been initialized and has a valid log in /var/lib/etcd
+etcdserver: discovery token ignored since a cluster has already been initialized. Valid log found at /var/lib/etcd
 ```
 
 # 0.4 to 0.5+ Migration Guide

etcdserver/server.go

@@ -264,7 +264,7 @@ func NewServer(cfg *ServerConfig) (*EtcdServer, error) {
 		id, n, s, w = startNode(cfg, cfg.Cluster.MemberIDs())
 	case haveWAL:
 		if cfg.ShouldDiscover() {
-			log.Printf("etcdserver: warn: ignoring discovery: etcd has already been initialized and has a valid log in %q", cfg.WALDir())
+			log.Printf("etcdserver: discovery token ignored since a cluster has already been initialized. Valid log found at %q", cfg.WALDir())
 		}
 		var index uint64
 		snapshot, err := ss.Load()