[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.
the only new section compared to #4627 in here is "Session Cookies and Caching"

but you are right that this is weird. i just reformatted this section, did not change the content. but will have another look into this.