spec: document invariants for actions without proofs

[redshiftzero]

In #3867 we documented in the protocol spec the integrity invariants of each action containing a proof. We should do the same for each action that does not contain a proof, documenting what part of the system upholds that invariant (e.g. validation in ActionHandler, etc.).

Actions without proofs:

• ValidatorDefinition
• Delegate
• Undelegate
• IbcRelay
• Ics20Withdrawal
• PositionOpen
• PositionClose
• PositionWithdraw
• PositionRewardClaim
• ProposalSubmit
• ProposalWithdraw
• ValidatorVote
• ProposalDepositClaim
• CommunityPoolSpend
• CommunityPoolOutput
• CommunityPoolDeposit
• ActionDutchAuctionSchedule
• ActionDutchAuctionEnd
• ActionDutchAuctionWithdraw

[redshiftzero]

Closely related to #4076, which documents the balance commitments on a per-action basis