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.
@dbu may go could use some parts of https://github.com/symfony/symfony-docs/pull/4628/files#r22298163 ? i just want to make sure that the reader get the idea that there a a bunch of caching strategies. filling up the varnish with user specific data is probably the worst one :)