[skip ci]Add comments for rmqID (#9419)

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

internal/util/mqclient/rmq_id.go

@@ -17,6 +17,7 @@ import (
 	"github.com/milvus-io/milvus/internal/util/rocksmq/server/rocksmq"
 )
 
+// rmqID wraps message ID for rocksmq
 type rmqID struct {
 	messageID rocksmq.UniqueID
 }