DOC Anonymise example QR code / key

Some users were occasionally scanning the sample QR code instead of
the code displayed in their UI, resulting in a broken TOTP entry
named after... me!

The QR code and secret key are now unusable, and the description
tries to make this as clear as possible.

Author: Cheddam

docs/en/userguide/01_User_manual/02_Using_authenticator_apps.md

@@ -23,9 +23,11 @@ screen, and press Next.
 
 Open your authentication app on your phone, and either:
 
-- Scan the displayed barcode with the app, or
+- Scan the displayed QR code with the app, or
 - Manually enter the displayed secret key into the app.
 
+(The QR code and secret key in the below screenshot are examples, and will not work.)
+
 ![A screenshot of the barcode and secret key shown in the Authenticator app setup flow](../_images/01-02-2-totp-scan.png)
 
 Once you have set up the app and can see codes being generated, press **Next**.