Staking improve payout_validator (#5208)

* add documentation and optimise payout_validator a bit

* bump spec version to be safe

this is just in case clipped exposure and exposure have different fields
own and total. This shouldn't be the case but better be safe in case
storage is wrong.

* Update frame/staking/src/lib.rs

Co-Authored-By: joe petrowski <[email protected]>

Co-authored-by: joe petrowski <[email protected]>