Add some explanation about overlapping instances
Add some explanation about overlapping instances

Trac #3734 suggested addding some extra guidance about 
incoherence and overlap; now done

ryantm/ghc,elieux/ghc,TomMD/ghc,snoyberg/ghc,nathyong/microghc-ghc,nathyong/microghc-ghc,nomeata/ghc,urbanslug/ghc,AlexanderPankiv/ghc,mettekou/ghc,hferreiro/replay,tjakway/ghcjvm,lukexi/ghc,frantisekfarka/ghc-dsi,urbanslug/ghc,lukexi/ghc-7.8-arm64,urbanslug/ghc,acowley/ghc,wxwxwwxxx/ghc,hferreiro/replay,acowley/ghc,wxwxwwxxx/ghc,AlexanderPankiv/ghc,oldmanmike/ghc,hferreiro/replay,ekmett/ghc,gcampax/ghc,lukexi/ghc,ezyang/ghc,spacekitteh/smcghc,nushio3/ghc,anton-dessiatov/ghc,mettekou/ghc,vikraman/ghc,urbanslug/ghc,elieux/ghc,wxwxwwxxx/ghc,olsner/ghc,TomMD/ghc,acowley/ghc,sdiehl/ghc,nathyong/microghc-ghc,frantisekfarka/ghc-dsi,frantisekfarka/ghc-dsi,mettekou/ghc,ml9951/ghc,fmthoma/ghc,ekmett/ghc,elieux/ghc,olsner/ghc,spacekitteh/smcghc,nushio3/ghc,siddhanathan/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,lukexi/ghc-7.8-arm64,ezyang/ghc,hferreiro/replay,snoyberg/ghc,jstolarek/ghc,green-haskell/ghc,acowley/ghc,ml9951/ghc,sgillespie/ghc,shlevy/ghc,hferreiro/replay,urbanslug/ghc,da-x/ghc,forked-upstream-packages-for-ghcjs/ghc,nathyong/microghc-ghc,ilyasergey/GHC-XAppFix,mcmaniac/ghc,spacekitteh/smcghc,jstolarek/ghc,mfine/ghc,GaloisInc/halvm-ghc,sgillespie/ghc,jstolarek/ghc,oldmanmike/ghc,shlevy/ghc,ekmett/ghc,mfine/ghc,vTurbine/ghc,christiaanb/ghc,acowley/ghc,ml9951/ghc,mfine/ghc,AlexanderPankiv/ghc,anton-dessiatov/ghc,da-x/ghc,fmthoma/ghc,mfine/ghc,ghc-android/ghc,frantisekfarka/ghc-dsi,wxwxwwxxx/ghc,nkaretnikov/ghc,nomeata/ghc,mettekou/ghc,snoyberg/ghc,vTurbine/ghc,nushio3/ghc,wxwxwwxxx/ghc,vTurbine/ghc,wxwxwwxxx/ghc,fmthoma/ghc,oldmanmike/ghc,ekmett/ghc,nathyong/microghc-ghc,tjakway/ghcjvm,christiaanb/ghc,nomeata/ghc,nomeata/ghc,TomMD/ghc,ghc-android/ghc,ezyang/ghc,sdiehl/ghc,nushio3/ghc,ghc-android/ghc,forked-upstream-packages-for-ghcjs/ghc,vTurbine/ghc,AlexanderPankiv/ghc,bitemyapp/ghc,bitemyapp/ghc,holzensp/ghc,ml9951/ghc,anton-dessiatov/ghc,gridaphobe/ghc,ezyang/ghc,elieux/ghc,nomeata/ghc,fmthoma/ghc,mettekou/ghc,anton-dessiatov/ghc,gcampax/ghc,da-x/ghc,GaloisInc/halvm-ghc,ryantm/ghc,ghc-android/ghc,mettekou/ghc,vikraman/ghc,oldmanmike/ghc,ilyasergey/GHC-XAppFix,spacekitteh/smcghc,green-haskell/ghc,ghc-android/ghc,vTurbine/ghc,mfine/ghc,sgillespie/ghc,ml9951/ghc,gcampax/ghc,anton-dessiatov/ghc,ml9951/ghc,TomMD/ghc,mfine/ghc,nkaretnikov/ghc,mcschroeder/ghc,tibbe/ghc,vTurbine/ghc,gridaphobe/ghc,nathyong/microghc-ghc,green-haskell/ghc,mettekou/ghc,da-x/ghc,tibbe/ghc,vTurbine/ghc,GaloisInc/halvm-ghc,nushio3/ghc,anton-dessiatov/ghc,mcschroeder/ghc,AlexanderPankiv/ghc,christiaanb/ghc,AlexanderPankiv/ghc,shlevy/ghc,elieux/ghc,elieux/ghc,tibbe/ghc,oldmanmike/ghc,tjakway/ghcjvm,snoyberg/ghc,lukexi/ghc-7.8-arm64,snoyberg/ghc,ryantm/ghc,GaloisInc/halvm-ghc,ghc-android/ghc,vikraman/ghc,TomMD/ghc,mcmaniac/ghc,shlevy/ghc,mcschroeder/ghc,urbanslug/ghc,fmthoma/ghc,forked-upstream-packages-for-ghcjs/ghc,mfine/ghc,forked-upstream-packages-for-ghcjs/ghc,christiaanb/ghc,sdiehl/ghc,gcampax/ghc,christiaanb/ghc,lukexi/ghc-7.8-arm64,ekmett/ghc,green-haskell/ghc,hferreiro/replay,shlevy/ghc,vikraman/ghc,mcmaniac/ghc,mcschroeder/ghc,vikraman/ghc,nkaretnikov/ghc,lukexi/ghc,tibbe/ghc,christiaanb/ghc,nushio3/ghc,jstolarek/ghc,da-x/ghc,olsner/ghc,fmthoma/ghc,forked-upstream-packages-for-ghcjs/ghc,elieux/ghc,nkaretnikov/ghc,mcschroeder/ghc,forked-upstream-packages-for-ghcjs/ghc,olsner/ghc,vikraman/ghc,oldmanmike/ghc,sgillespie/ghc,sgillespie/ghc,siddhanathan/ghc,sdiehl/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,lukexi/ghc,TomMD/ghc,tjakway/ghcjvm,TomMD/ghc,acowley/ghc,holzensp/ghc,acowley/ghc,nushio3/ghc,siddhanathan/ghc,bitemyapp/ghc,gcampax/ghc,da-x/ghc,mcmaniac/ghc,sgillespie/ghc,sdiehl/ghc,urbanslug/ghc,mcschroeder/ghc,bitemyapp/ghc,ml9951/ghc,sdiehl/ghc,tjakway/ghcjvm,gridaphobe/ghc,siddhanathan/ghc,holzensp/ghc,forked-upstream-packages-for-ghcjs/ghc,gridaphobe/ghc,tibbe/ghc,siddhanathan/ghc,fmthoma/ghc,AlexanderPankiv/ghc,ryantm/ghc,jstolarek/ghc,tjakway/ghcjvm,sdiehl/ghc,nathyong/microghc-ghc,snoyberg/ghc,ryantm/ghc,nkaretnikov/ghc,ezyang/ghc,ilyasergey/GHC-XAppFix,ilyasergey/GHC-XAppFix,vikraman/ghc,hferreiro/replay,da-x/ghc,holzensp/ghc,shlevy/ghc,olsner/ghc,GaloisInc/halvm-ghc,gcampax/ghc,frantisekfarka/ghc-dsi,olsner/ghc,olsner/ghc,siddhanathan/ghc,gridaphobe/ghc,tjakway/ghcjvm,gridaphobe/ghc,lukexi/ghc,ml9951/ghc,christiaanb/ghc,gridaphobe/ghc,shlevy/ghc,bitemyapp/ghc,ezyang/ghc,ezyang/ghc,gcampax/ghc,lukexi/ghc-7.8-arm64,mcmaniac/ghc,sgillespie/ghc,holzensp/ghc,green-haskell/ghc,anton-dessiatov/ghc,nkaretnikov/ghc,wxwxwwxxx/ghc,siddhanathan/ghc,ghc-android/ghc,nkaretnikov/ghc,spacekitteh/smcghc,oldmanmike/ghc