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.
i dont think we should suggest varying based on cookies. this is just useable if you vary on flags (Terms, Users Age, ...)