[skip ci]Add comment for rocksmq producer client (#13533)

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

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

@@ -28,6 +28,7 @@ type producer struct {
 	topic string
 }
 
+// newProducer creates a rocksmq producer from options
 func newProducer(c *client, options ProducerOptions) (*producer, error) {
 	if c == nil {
 		return nil, newError(InvalidConfiguration, "client is nil")