[Cookbook][Security] Added doc for x509 pre authenticated listener

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

This can be merged in 2.3. Regards.
How about:

>
You will need to create (or use) a "user provider" that turns that username into a User object if your choice.