This method allows you mark an existing notification as seen which means it will no longer be returned from the unseen API or within the web interface.
This method can only be used by user access tokens. Application wide tokens do not have access to this method.
id - the ID of the notification you wish to mark as seen (optional - if ommitted all notifications for user will be marked as read)
This method will return the notification data as shown on the unseen notifications page.