* Move `IsSubType` and write some docs for the trait

This moves the `IsSubType` trait from dispatch.rs to traits.rs. It also
adds docs to make the trait better understandable.

* Update frame/support/src/traits.rs

Co-authored-by: Shawn Tabrizi <[email protected]>

Co-authored-by: Shawn Tabrizi <[email protected]>