Add comment.

fcm.go

@@ -426,6 +426,7 @@ type MessageMultiSendResponse struct {
 	Failed    int
 }
 
+// Запрос на отправку пуш-оповещения.
 type SendRequest struct {
 	// Flag for testing the request without actually delivering the message.
 	ValidateOnly bool `json:"validate_only,omitempty"`