Documentation: Clarify etcd security documentation

I set up all the security features in etcd in my sandbox cluster recently and felt that the documentation was a bit lacking, so this commit rewrites large parts of it.

You can see this rendered at: https://github.com/tazjin/etcd/blob/etcd-security-docs/Documentation/security.md

It's supposed to be structured as a *base introduction* -> *clear examples* -> *FAQ*.

The first FAQ question is the same that was in the old documentation - if somebody could check whether that issue is still relevant that'd be great (I personally haven't run into it).

Please comment if you think it could be improved or with other feedback.

//V