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.
you could also store the username in a cookie / localstorage or something else to print it. from my point of view storing user specific content in the reverse proxy can be a good idea but most time you should try to avoid it. people could think it's a good idea to mess up the cache with a Vary on the Cookie. This issue is hard to find for newbies because most times, if they test they use the same cookie. Why not creating a advanced cookbook where we explain how to cache based on Cookies (Javascript), Cookies + Vary (Varnish), Localstorage, ....