chore: add some annotations (#1544)

ref: #1075 
because I am not a native English, maybe have a bit problem.

render/data.go

@@ -6,6 +6,7 @@ package render
 
 import "net/http"
 
+// Data contains ContentType and bytes data.
 type Data struct {
 	ContentType string
 	Data        []byte
@@ -18,6 +19,7 @@ func (r Data) Render(w http.ResponseWriter) (err error) {
 	return
 }
 
+// WriteContentType (Data) writes custom ContentType.
 func (r Data) WriteContentType(w http.ResponseWriter) {
 	writeContentType(w, []string{r.ContentType})
 }