Document consensus rules from Zcash spec: 4.12 Balance (Sprout)

[mpguerra]

For each consensus rule:

• check that the rule is implemented in Zebra
• make sure the latest version of the rule is quoted in the docs for the code that implements it
• make sure the code implements the latest version of the rule

Consensus rule

https://zips.z.cash/protocol/protocol.pdf#joinsplitbalance

See ZIP 209: Prohibit Negative Shielded Chain Value Pool Balances also

Implementation

• ZIP-209: Validate transaction value balances - Implementation #2381
  • PR: Check remaining transaction value & make value balance signs match the spec  #2566

[teor2345]

It doesn't really make sense for oxarbitrage to do two of the negative value balance rules, and me to do the other one. I'm pretty sure all 3 rules will get documented in exactly the same module. (Maybe even in the same method?)

I'm happy to pick up other tickets though.