Doc: warn about tampered server nonce

https://github.com/borgbackup/borg/pull/6188#discussion_r794752672
> Well, guess one could also use max(list of trusted nonce values).
>
> The real issue is if you have lost all or some of the trusted
> (client side) nonce values and you also have reason to not trust the
> server side nonce, because someone might attack you on the server.

docs/faq.rst

@@ -707,7 +707,7 @@ on one machine.
 
 For repositories that are used by multiple users and/or from multiple machines
 it is safest to avoid running *any* commands that modify the repository after
-the nonce is deleted.
+the nonce is deleted or if you suspect it may have been tampered with. See :ref:`attack_model`.
 
 Common issues
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