[skip ci]Add comment for rocksmq producer client (#13534)

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

internal/util/rocksmq/client/rocksmq/producer_impl.go

@@ -44,6 +44,7 @@ func newProducer(c *client, options ProducerOptions) (*producer, error) {
 	}, nil
 }
 
+// Topic returns the topic name of producer
 func (p *producer) Topic() string {
 	return p.topic
 }