Make Pay trait from salaries pallet more generic (#13609)

* Make Pay trait from salaries pallet more generic

* Rename and add missing

* Update frame/support/src/traits/tokens/pay.rs

* Update pay.rs

* Update pay.rs

* Update pay.rs

* Add better documentation for the AssetKind associated type

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Co-authored-by: Gavin Wood <gavin@parity.io>
Co-authored-by: parity-processbot <>