docs(providers): add tip about async provider code

[ndom91]

Reasoning 💡

Adds tip regarding using async code in ones provider instantiation. Wasn't sure if this was the best place to put this, but it seemed appropriate enough. Im open to any other suggestions though obviously 👍

Checklist 🧢

• Documentation
• [ ] Tests
• Ready to be merged

Affected issues 🎟

Related: #2394

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[sastraxi]

Perhaps I'm being daft, but shouldn't the async function return NextAuth(...? Right now it's just instantiating it and throwing the value away.

[balazsorban44]

It's a Next.js API route handler, nothing needs to be returned. It uses res.{json,end,send} to finish the response, that's why you send req and res to it. return might be the way in other frameworks like SvelteKit though, but we aren't there yet. (#2294)

[sastraxi]

@balazsorban44 ahh right, forgot the API route handlers work just like express. Thanks for the clarification!