Update and improve documentation in Data.Foldable
Update and improve documentation in Data.Foldable

Previously there were a few obsolete references to `Data.List` and
the descriptions were lacking examples.

Fixes #11065.

Test Plan: Read it.

Reviewers: ekmett, goldfire, hvr, austin

Reviewed By: hvr

Subscribers: nomeata, thomie

Differential Revision: https://phabricator.haskell.org/D1617

GHC Trac Issues: #11065

vikraman/ghc,nushio3/ghc,tjakway/ghcjvm,oldmanmike/ghc,nushio3/ghc,nushio3/ghc,oldmanmike/ghc,olsner/ghc,gridaphobe/ghc,sgillespie/ghc,shlevy/ghc,sdiehl/ghc,tjakway/ghcjvm,mcschroeder/ghc,oldmanmike/ghc,snoyberg/ghc,sdiehl/ghc,gridaphobe/ghc,snoyberg/ghc,oldmanmike/ghc,snoyberg/ghc,mcschroeder/ghc,mcschroeder/ghc,sdiehl/ghc,mettekou/ghc,tjakway/ghcjvm,vTurbine/ghc,ezyang/ghc,tjakway/ghcjvm,vikraman/ghc,vikraman/ghc,ezyang/ghc,olsner/ghc,vTurbine/ghc,sgillespie/ghc,snoyberg/ghc,shlevy/ghc,ezyang/ghc,sgillespie/ghc,vikraman/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,mettekou/ghc,vTurbine/ghc,oldmanmike/ghc,mettekou/ghc,olsner/ghc,nushio3/ghc,nushio3/ghc,tjakway/ghcjvm,shlevy/ghc,oldmanmike/ghc,shlevy/ghc,mcschroeder/ghc,mcschroeder/ghc,mcschroeder/ghc,vikraman/ghc,gridaphobe/ghc,vikraman/ghc,gridaphobe/ghc,sdiehl/ghc,olsner/ghc,sgillespie/ghc,snoyberg/ghc,sgillespie/ghc,tjakway/ghcjvm,gridaphobe/ghc,ezyang/ghc,sdiehl/ghc,sgillespie/ghc,mettekou/ghc,vTurbine/ghc,vTurbine/ghc,olsner/ghc,mettekou/ghc,sgillespie/ghc,olsner/ghc,olsner/ghc,snoyberg/ghc,gridaphobe/ghc,vikraman/ghc,tjakway/ghcjvm,mettekou/ghc,snoyberg/ghc,oldmanmike/ghc,mcschroeder/ghc,vTurbine/ghc,sdiehl/ghc,nushio3/ghc,nushio3/ghc,gridaphobe/ghc,shlevy/ghc,mettekou/ghc,vTurbine/ghc