[skip ci]Add comments for RegisterConsumer (#9912)

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

internal/util/rocksmq/server/rocksmq/rocksmq_impl.go

@@ -371,6 +371,7 @@ func (rmq *rocksmq) CreateConsumerGroup(topicName, groupName string) error {
 	return nil
 }
 
+// RegisterConsumer registers a consumer in rocksmq consumers
 func (rmq *rocksmq) RegisterConsumer(consumer *Consumer) {
 	start := time.Now()
 	if vals, ok := rmq.consumers.Load(consumer.Topic); ok {