* `pallet-staking`: Expose missing consts

* Apply suggestions from code review

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

* Update the source docs

Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Nikolay Volf <[email protected]>
Co-authored-by: joe petrowski <[email protected]>