Document Safe Haskell restrictions on Generic instances
Document Safe Haskell restrictions on Generic instances

Commit 578fbeca31dd3d755e24e910c3a7327f92bc4ee3 disallowed hand-written
`Generic` instances in Safe Haskell, but this wasn't documented anywhere.

shlevy/ghc,shlevy/ghc,shlevy/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,snoyberg/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,snoyberg/ghc,sdiehl/ghc,olsner/ghc,sdiehl/ghc,olsner/ghc,snoyberg/ghc,mettekou/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,mettekou/ghc,sdiehl/ghc,shlevy/ghc,snoyberg/ghc,olsner/ghc,olsner/ghc,snoyberg/ghc,mettekou/ghc,ezyang/ghc,ezyang/ghc,mettekou/ghc,snoyberg/ghc,olsner/ghc,sdiehl/ghc,mettekou/ghc,ezyang/ghc,sdiehl/ghc,snoyberg/ghc