Document how to generate strong OpenPGP keys with Seahorse instead of Claws Mail

Reported by user, confirmed by me:

If chosen when configuring GPG support for the account, the button ‘Generate a new key pair’ will generate a 1024 bit DSA
key, without giving any possibility of changing the type of key or the size.

Parent Task: #5316

Related issues

Related to #9422 (closed)
Related to #6568

Original created by @emmapeel on 9404 (Redmine)

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