Add some documentation about type-level literals.
Add some documentation about type-level literals.

I moved the "promoted literals" sub-section into a separate section,
as many folks were not finding the docs.  I also added some additional
paragraphs describing the current state of the feature.

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