[skip ci]Add comment for rocksmq reader (#13014)

Signed-off-by: fishpenguin <kun.yu@zilliz.com>

internal/util/mqclient/rmq_reader.go

@@ -17,6 +17,7 @@ func (rr *rmqReader) Topic() string {
 	return rr.r.Topic()
 }
 
+// Next returns the next message of reader, blocking until a message is available
 func (rr *rmqReader) Next(ctx context.Context) (Message, error) {
 	rMsg, err := rr.r.Next(ctx)
 	if err != nil {