Update response-helpers.md (#42378)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)


Co-authored-by: Balázs Orbán <18369201+balazsorban44@users.noreply.github.com>
Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/api-routes/response-helpers.md

@@ -98,6 +98,8 @@ export default function handler(
 }
 ```
 
+> Note: The body of `NextApiRequest` is `any` because the client may include any payload. You should validate the type/shape of the body at runtime before using it.
+
 To view more examples using types, check out the [TypeScript documentation](/docs/basic-features/typescript.md#api-routes).
 
 If you prefer to view your examples within a real projects structure you can checkout our examples repository: