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 <25483142+joepetrowski@users.noreply.github.com>

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

substrate/bin/node/runtime/src/lib.rs

@@ -82,7 +82,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	// and set impl_version to 0. If only runtime
 	// implementation changes and behavior does not, then leave spec_version as
 	// is and increment impl_version.
-	spec_version: 234,
+	spec_version: 235,
 	impl_version: 0,
 	apis: RUNTIME_API_VERSIONS,
 };