Clarification of the purpose of Tidying (in GHC.Iface.Tidy)

Closed Bartłomiej Cieślar requested to merge barci2/ghc-dev:tidy-purpose-clarification into master

This MR simply adds a top-level comment describing the purpose of GHC.Iface.Tidy and lifts some information out from the Note [About the NameSorts] to the constructors of NameSort

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