Describe PeerSets behavior at a network upgrade's activation height

[jvff]

Motivation

ZIP-201 describes how a Zcash node should behave when it reaches a network upgrade activation height. Zebra doesn't implement all the details specified there, so we need to document what it does implement and what it doesn't and why.

This closes #706.

Specifications

The motivation for ZIP-201 is:

With scheduled network upgrades, at the activation height, nodes on each consensus branch should disconnect from nodes on other consensus branches and only accept new incoming connections from nodes on the same consensus branch.

Solution

The specification itself contains many details that are zcashd specific, so in this PR we document the behavior of zebra and how it satisfies the original motivation for ZIP-201, while also describing why other details don't need to be implemented.

Review

@teor2345 can review this, since they have recently reviewed the implementation code in #3108.

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour
• Tests for Errors

[teor2345]

Looks good, we could merge this as it is.

But we might want to describe the number of connected peers in a bit more detail.

[teor2345]

Looks good!