rafthttp: clarify "became inactive" warning

Signed-off-by: Gyuho Lee <leegyuho@amazon.com>

rafthttp/peer_status.go

@@ -56,7 +56,7 @@ func (s *peerStatus) deactivate(failure failureType, reason string) {
 	msg := fmt.Sprintf("failed to %s %s on %s (%s)", failure.action, s.id, failure.source, reason)
 	if s.active {
 		plog.Errorf(msg)
-		plog.Infof("peer %s became inactive", s.id)
+		plog.Infof("peer %s became inactive (message send to peer failed)", s.id)
 		s.active = false
 		s.since = time.Time{}
 		return