[skip ci] Add comment for EventTypeCode (#13368)

Signed-off-by: Nshaoyue.chen <shaoyue.chen@zilliz.com>

internal/storage/event_writer.go

@@ -22,8 +22,10 @@ import (
 	"github.com/milvus-io/milvus/internal/proto/schemapb"
 )
 
+// EventTypeCode represents event type by code
 type EventTypeCode int8
 
+// EventTypeCode definitions
 const (
 	DescriptorEventType EventTypeCode = iota
 	InsertEventType
@@ -36,6 +38,7 @@ const (
 	EventTypeEnd
 )
 
+// String returns the string representation
 func (code EventTypeCode) String() string {
 	codes := map[EventTypeCode]string{
 		DescriptorEventType:       "DescriptorEventType",