Document how to allow macOS Startup Security Utility to boot on external media

It could prevent from starting on an external media:

https://support.apple.com/en-us/HT208198

The following error message was reported to us (in French):

« Les réglages de sécurité ne permettent pas à ce Mac d’utiliser un disque de démarrage externe. »

This should affect by default, as per https://support.apple.com/en-us/HT208862:

• iMac Pro
• Mac mini models introduced in 2018
• MacBook Air models introduced in 2018
• MacBook Pro models introduced in 2018

It would be good to:

• Have our help desk confirm that this is really happening on such machines by default
• Test one of these machines:
  • What’s happening by default?
  • Does allowing booting from external media works?
• Include this in our installation instructions for macOS

Related issues

• Blocks #16711

Original created by @sajolida on 16410 (Redmine)