[Cookbook][Cache] Added config example for Varnish 4.0

| Q             | A
| ------------- | ---
| Doc fix?      | [yes]
| New docs?     | [no] 
| Applies to    | [all]
| Fixed tickets | []

After upgrading to Varnish 4.0, I've noticed the documentation was not really up-to-date anymore:

 * according to https://www.varnish-cache.org/docs/4.0/whats-new/upgrading.html#req-not-available-in-vcl-backend-response,
   
  `beresp` is available in `vcl_backend_response`

 * according to https://www.varnish-cache.org/docs/4.0/whats-new/upgrading.html#hit-for-pass-objects-are-created-using-beresp-uncacheable,

   `hit_for_pass` objects are created via `beresp.uncacheable`

Could such addition be enough?
perhaps you want to make this // instead or a full docblock