[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.
In practical terms, what should this mean to the user? I think it means that you're telling Varnish to *yes* be ok with caching requests that contain cookies. And so, as a user, you must be very careful to *not* rely on the session for any parts of your page where you return caching headers. If you violate this rule, then you will cache something with - for example - a user's username in it.

Is this about right? I want to walk the user through this as much as possible. For me, it seems that you should be (as you elude to here) identifying which portions of your page need the session, isolating them into non-cached ESI fragments, then caching the whole page (or doing a complete opposite strategy where you isolate non-session-needing heavy pieces, then don't cache the whole page).

The point is, this is non-trivial, so perhaps we need to walk them through a bit more tutorial-styled.