QtIFW

Doc: update the Register File Extension example

And the docs.

Change-Id: I35f5edc1ad95df29d033e4220af003bac480d6e3
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>
Reviewed-by: Niels Weber <niels.weber@theqtcompany.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>