chore: add some annotations (#1544)

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

render/xml.go

@@ -9,17 +9,20 @@ import (
 	"net/http"
 )
 
+// XML contains the given interface object.
 type XML struct {
 	Data interface{}
 }
 
 var xmlContentType = []string{"application/xml; charset=utf-8"}
 
+// Render (XML) encodes the given interface object and writes data with custom ContentType.
 func (r XML) Render(w http.ResponseWriter) error {
 	r.WriteContentType(w)
 	return xml.NewEncoder(w).Encode(r.Data)
 }
 
+// WriteContentType (XML) writes XML ContentType for response.
 func (r XML) WriteContentType(w http.ResponseWriter) {
 	writeContentType(w, xmlContentType)
 }