Doc Thunderbird 78 not able to import PGP private key with offline master key

As was noted in #17147 (closed):

Offline master key is not supported yet. But presumably the sort of users who do that can enable mail.openpgp.allow_external_gnupg themselves.

I didn't know this when I tried to import my private key, which has an offline master key.

The error message given by Thunderbird doesn't mention anything about Thunderbird not being able to import private keys that have their master keys offline. The error message given by Thunderbird led me to believe I was repeatedly entering my passphrase incorrectly.

I think it's worthwhile to document this bug and how to work around it, namely, by setting the mail.openpgp.allow_external_gnupg preference to true.

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