Change pure functions to functions (#27192)

The right terminology should be used so we don't learn new developers wrong facts.

Using pure function here is plain wrong, the definition of a pure functions is as follows:

> The function return values are identical for identical arguments. The function application has no side effects. 

What you have here both have side effects, the calls to `res.setXXX`, and it isn't identical for identical arguments since you have `Date.now()`.



## Bug

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

## Documentation / Examples

- [ ] Make sure the linting passes

docs/api-routes/api-middlewares.md

@@ -119,7 +119,7 @@ export default handler
 
 ## Extending the `req`/`res` objects with TypeScript
 
-For better type-safety, it is not recommended to extend the `req` and `res` objects. Instead, use pure functions to work with them:
+For better type-safety, it is not recommended to extend the `req` and `res` objects. Instead, use functions to work with them:
 
 ```ts
 // utils/cookies.ts