Improve documentation for type wildcards
Improve documentation for type wildcards

This docmentation improvement was triggered by Trac #12072

ezyang/ghc,ezyang/ghc,ezyang/ghc,vikraman/ghc,snoyberg/ghc,olsner/ghc,vTurbine/ghc,olsner/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,snoyberg/ghc,sgillespie/ghc,mettekou/ghc,shlevy/ghc,vikraman/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sgillespie/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,vikraman/ghc,sgillespie/ghc,sgillespie/ghc,sdiehl/ghc,vikraman/ghc,vTurbine/ghc,vikraman/ghc,snoyberg/ghc,vTurbine/ghc,shlevy/ghc,olsner/ghc,vTurbine/ghc,ezyang/ghc,sdiehl/ghc,vTurbine/ghc,snoyberg/ghc,ezyang/ghc,vTurbine/ghc,vikraman/ghc,olsner/ghc,snoyberg/ghc,sdiehl/ghc,mettekou/ghc,olsner/ghc,sgillespie/ghc,mettekou/ghc,sgillespie/ghc,mettekou/ghc,ezyang/ghc,shlevy/ghc,vikraman/ghc,snoyberg/ghc,vTurbine/ghc,shlevy/ghc,snoyberg/ghc,sgillespie/ghc,mettekou/ghc,mettekou/ghc