docs: add invariants to module specs

[tac0turtle]

Invariants are a key factor is using modules and modifying them, but its hard to know what the invariants are for a module. We should document them in the module level specs.

[ValarDragon]

Should also include invariants that are assumed but not actually tested in simulation

[Wryhder]

@tac0turtle I'd like to work on this.

To confirm:

1. This is the list of module specs?
2. Could you let me know how much documentation is needed? For example, for the staking module, there seem to be four invariants (ModuleAccountInvariants, NonNegativePowerInvariant, PositiveDelegationInvariant, DelegatorSharesInvariant)?
   • Am I correct, and this is how to find the invariants for each module?
   • Is listing them and adding short descriptive comments enough, or would there be more detail?
3. What does @ValarDragon mean by "invariants that are assumed but not actually tested in simulation"?

[tac0turtle]

closing this as its getting reworked with the simulator