document the critical properties / invariants of each proof statement and why they are upheld

[redshiftzero]

We should add to the protocol docs a description of the invariants of each proof statement. From discussion with @hdevalence:

an example high-level property/invariant would be “notes cannot be spent without spend authority” and the explanation would cite each of the proof statements and how they fit together

After discussion with @cronokirby, we are going to:

• [x] Define the invariants with respect to the integrity of the state of the system and the allowable transitions (e.g. a transition should not create/destroy funds) for Spends / Outputs
• [x] Ensure the description of the proof statements in the Spend / Output section of the protocol docs are accurate
• [x] Write down why each invariant is upheld by the Spend / Output proof statements or another part of the protocol
• [x] Perform the same steps for the rest of the actions