docs: overhaul Derive{Functor,Foldable,Traversable} notes
docs: overhaul Derive{Functor,Foldable,Traversable} notes

The previous users' guide documentation was too implementation-oriented. This
attempts to make it more accessible to those who aren't familiar with how
`-XDeriveFunctor` and friends work (and more importantly, what will not work
when using them).

Fixes #10831.

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D1293

GHC Trac Issues: #10831

vikraman/ghc,sgillespie/ghc,da-x/ghc,mcschroeder/ghc,snoyberg/ghc,gridaphobe/ghc,AlexanderPankiv/ghc,sdiehl/ghc,elieux/ghc,elieux/ghc,mcschroeder/ghc,siddhanathan/ghc,gridaphobe/ghc,sgillespie/ghc,tjakway/ghcjvm,GaloisInc/halvm-ghc,nushio3/ghc,mcschroeder/ghc,tjakway/ghcjvm,sdiehl/ghc,da-x/ghc,oldmanmike/ghc,vTurbine/ghc,shlevy/ghc,ezyang/ghc,siddhanathan/ghc,mettekou/ghc,ezyang/ghc,AlexanderPankiv/ghc,tjakway/ghcjvm,elieux/ghc,oldmanmike/ghc,shlevy/ghc,elieux/ghc,vikraman/ghc,sdiehl/ghc,mettekou/ghc,nushio3/ghc,ezyang/ghc,siddhanathan/ghc,siddhanathan/ghc,mettekou/ghc,sdiehl/ghc,snoyberg/ghc,mettekou/ghc,snoyberg/ghc,oldmanmike/ghc,siddhanathan/ghc,sgillespie/ghc,sdiehl/ghc,da-x/ghc,oldmanmike/ghc,sgillespie/ghc,vTurbine/ghc,gridaphobe/ghc,siddhanathan/ghc,shlevy/ghc,olsner/ghc,AlexanderPankiv/ghc,da-x/ghc,elieux/ghc,mettekou/ghc,shlevy/ghc,shlevy/ghc,sgillespie/ghc,shlevy/ghc,elieux/ghc,tjakway/ghcjvm,nushio3/ghc,vTurbine/ghc,olsner/ghc,vikraman/ghc,ezyang/ghc,ezyang/ghc,AlexanderPankiv/ghc,da-x/ghc,vTurbine/ghc,AlexanderPankiv/ghc,sdiehl/ghc,tjakway/ghcjvm,snoyberg/ghc,shlevy/ghc,olsner/ghc,GaloisInc/halvm-ghc,sdiehl/ghc,oldmanmike/ghc,nushio3/ghc,gridaphobe/ghc,ezyang/ghc,mettekou/ghc,mcschroeder/ghc,vTurbine/ghc,olsner/ghc,gridaphobe/ghc,nushio3/ghc,olsner/ghc,da-x/ghc,da-x/ghc,GaloisInc/halvm-ghc,vikraman/ghc,sgillespie/ghc,vTurbine/ghc,snoyberg/ghc,vikraman/ghc,nushio3/ghc,vikraman/ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,gridaphobe/ghc,AlexanderPankiv/ghc,ezyang/ghc,elieux/ghc,siddhanathan/ghc,mettekou/ghc,snoyberg/ghc,mcschroeder/ghc,tjakway/ghcjvm,nushio3/ghc,snoyberg/ghc,oldmanmike/ghc,gridaphobe/ghc,mcschroeder/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,olsner/ghc,vikraman/ghc,vTurbine/ghc,GaloisInc/halvm-ghc,oldmanmike/ghc,tjakway/ghcjvm,AlexanderPankiv/ghc,sgillespie/ghc,olsner/ghc