[WIP] Varnish cookbook session cookie handling

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #3881

This builds on top of #4627 but i wanted to keep it separate as there are open questions in here.
Actually, I still mean everything I said, but most of what I said should be in the book. I still think we can add a few more details, like those I have in my first paragraph.