Add comments & notes explaining the typing of pattern synonym definitions
Add comments & notes explaining the typing of pattern synonym definitions

(cherry picked from commit d2c4f9758ca735f294033401efef225699c292f8)

hferreiro/replay,lukexi/ghc-7.8-arm64,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,hferreiro/replay,lukexi/ghc-7.8-arm64,hferreiro/replay,lukexi/ghc-7.8-arm64