Description about designated redemptor and expiration date
更新於：
Anyone can redeem the code if you didn't give the expiration date and recipient.
If you only specify the ID of recipient, only you and the recipient can redeem the code.
If you only set up the expiration date, anyone besides you can redeem the code before the expiration date, after the date it can only be redeemed by yourself.1
If you specify the ID of the recipient and the expiration date, only the recipient can redeem the code before the expiration date. After the expiration date, the code can only be redeemed by yourself.
