Improve documentation when MAC spoofing is disabled automatically

Right now the fact that a network interface might be disabled automatically when MAC spoofing fails is only mentioned very briefly in the section “When to disable MAC address spoofing”.

I’m afraid that this behavior of Tails might be very hard to understand for people, even when the ones reading our documentation carefully.

We should at least add a screenshot of the current error message and maybe do other smart things…

Related issues

Blocks #15941 (closed)

Original created by @sajolida on 16369 (Redmine)

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