Better document TypeRep patterns
Better document TypeRep patterns

As pointed out in #14199 these are rather non-trivial; extra
documentation is in order.

[skip ci]

Test Plan: Read it

Reviewers: dfeuer, austin, hvr

Subscribers: rwbarton, thomie

GHC Trac Issues: #14199

Differential Revision: https://phabricator.haskell.org/D3943

ezyang/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc