callback: add missing struct fields (#647)

* fix: callback struct fields

* fix: documentation

callback.go

@@ -26,6 +26,15 @@ type Callback struct {
 	// a bad client can send arbitrary data in this field.
 	Data string `json:"data"`
 
+	// ChatInstance is a global identifier, uniquely corresponding to
+	// the chat to which the message with the callback button was sent.
+	ChatInstance string `json:"chat_instance"`
+
+	// GameShortName is a unique identifier of the game for which a URL
+	// is requested from the bot when a user presses the Play button of
+	// that game. GameShortName may be empty
+	GameShortName string `json:"game_short_name"`
+
 	// Unique displays an unique of the button from which the
 	// callback was fired. Sets immediately before the handling,
 	// while the Data field stores only with payload.