DOC: config: add a few more explanation in http-reusee regarding sni-auto

DOC: config: add a few more explanation in http-reusee regarding sni-auto

The default sni-auto that aims at not upsetting certain servers doing
excessive checks of SNI vs host has some drawbacks (lower reuse ratio)
that are particularly hard to diagnose, so let's explain how connections
are reused/purged when dealing with many hosts, and how to cheat as well.

Let's also mention the expression used by "sni-auto" since it was only
mentioned in the code.