Rewrite the varnish cookbook article

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #1505, #4176
Or have only Varnish < 4 tabs and write above them something like: ‘If you use a Varnish version older than 4, you need to make some changes to fix the default behaviour.’