chore: add some annotations (#1544)

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

render/json_17.go

@@ -12,10 +12,12 @@ import (
 	"github.com/gin-gonic/gin/internal/json"
 )
 
+// PureJSON contains the given interface object.
 type PureJSON struct {
 	Data interface{}
 }
 
+// Render (PureJSON) writes custom ContentType and encodes the given interface object.
 func (r PureJSON) Render(w http.ResponseWriter) error {
 	r.WriteContentType(w)
 	encoder := json.NewEncoder(w)
@@ -23,6 +25,7 @@ func (r PureJSON) Render(w http.ResponseWriter) error {
 	return encoder.Encode(r.Data)
 }
 
+// WriteContentType (PureJSON) writes custom ContentType.
 func (r PureJSON) WriteContentType(w http.ResponseWriter) {
 	writeContentType(w, jsonContentType)
 }