chore(docs): Add details about linking accounts when user is already signed in

[mbrookson]

☕️ Reasoning

Update documentation to add details about linking accounts when authenticating via other providers when a user is already signed in.

See comment here: https://github.com/nextauthjs/next-auth/discussions/990#discussioncomment-241702

🧢 Checklist

• [x] Documentation
• [ ] Tests
• [ ] Ready to be merged

📌 Resources

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Name	Status	Preview	Comments	Updated (UTC)
auth-docs	❌ Failed (Inspect)			May 14, 2024 9:04pm
next-auth-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 14, 2024 9:04pm

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