[FAQ] New entry for re-used sync folder errors

[michaelstingl]

6.0 clients do a new check:

• Prevent: Same default sync folder for multiple accounts client#11204
• Spaces: Prevent same default sync folder for multiple accounts client#11364

@erikjv could you contribute a new paragraph here?
https://doc.owncloud.com/desktop/next/faq.html#error-messages

/cc @TheOneRing @HanaGemela