Clarify and document the peer scoring mechanism

[kamikazechaser]

From the docs:

Peers may be banned if their performance score is too low, or if an objectively malicious action was detected.

However the scoring mechanism is not described. Additionally, it would be good for node operators to understand:

1. How to keep the peer score up
2. How the score itself is determined
3. How to resolve a continuous peer ban
4. What are the implications/effects of a continuous peer ban