[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.
We occasionally (but consistently) have questions about this. Related items are #2564 and #2607. So yes, if it makes sense to tell the user to "try to only start the session when they need to",  then we could link there. To make things easier, we could avoid adding that link now (just add the note), then put some focus onto #2607 later.