staking: update the protocol specification

[erwanor]

Is your feature request related to a problem? Please describe.
The staking section of the protocol specification is severely out of date. We should actualize it so that it can be helpful for someone trying to understand what are the stated goals and general mechanisms of the staking component.

[conorsch]

Crucially, no code changes required here. This is about updating docs. Since we have an ongoing audit of these components right now, it'd be great to have fresh docs for the auditors. However, we're not blocking on this in order to ship 65 (#3554).