Add path for information in cookie setting (#42146)

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## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
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## Documentation / Examples

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docs/api-routes/request-helpers.md

@@ -128,7 +128,8 @@ import { setCookie } from '../../utils/cookies'
 
 const handler = (req: NextApiRequest, res: NextApiResponse) => {
   // Calling our pure function using the `res` object, it will add the `set-cookie` header
-  setCookie(res, 'Next.js', 'api-middleware!')
+  // Add the `set-cookie` header on the main domain and expire after 30 days
+  setCookie(res, 'Next.js', 'api-middleware!', { path: '/', maxAge: 2592000 })
   // Return the `set-cookie` header so we can display it in the browser and show that it works!
   res.end(res.getHeader('Set-Cookie'))
 }