[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.
I don't know exactly what use-case there is for the credentials in the ``PreAuthenticatedToken`` class, but I thought it would be great to document this feature because my PR on REMOTE_USER is also based on the ``PreAuthenticatedAuthenticationProvider`` and these features are not really documented. 

I think @fabpot implemented the X509 auth and might be able to answer that question.