Rewrite documentation for `Distribution.License`.
Rewrite documentation for `Distribution.License`.

* Fix copyright field.
* List valid versions for each license.
* Remove descriptions for most licenses; just link to them instead.
* Explain why BSD4 should be avoided.
* State that public domain is not a license.
* Link to user guide.
beni55/cabal,corngood/cabal,rimmington/cabal,fugyk/cabal,gbaz/cabal,anton-dessiatov/cabal,mmakowski/cabal,triplepointfive/cabal,mydaum/cabal,Helkafen/cabal,seereason/cabal,mmakowski/cabal,randen/cabal,edsko/cabal,kosmikus/cabal,typelead/epm,garetxe/cabal,kolmodin/cabal,seereason/cabal,thomie/cabal,bennofs/cabal,trskop/cabal,kolmodin/cabal,Heather/cabal,Peaker/cabal,enolan/cabal,sopvop/cabal,martinvlk/cabal,valderman/cabal,trskop/cabal,valderman/cabal,sopvop/cabal,ian-ross/cabal,mightybyte/cabal,sopvop/cabal,triplepointfive/cabal,randen/cabal,ghc-android/cabal,lukexi/cabal,thoughtpolice/cabal,kosmikus/cabal,headprogrammingczar/cabal,rimmington/cabal,x-y-z/cabal,fugyk/cabal,eric-stanley/cabal,themoritz/cabal,thoughtpolice/cabal,plow-technologies/cabal,gridaphobe/cabal,beni55/cabal,ghc-android/cabal,mightybyte/cabal,gbaz/cabal,anton-dessiatov/cabal,martinvlk/cabal,thomie/cabal,themoritz/cabal,x-y-z/cabal,Helkafen/cabal,bennofs/cabal,christiaanb/cabal,mydaum/cabal,plumlife/cabal,themoritz/cabal,enolan/cabal,typelead/epm,edsko/cabal,christiaanb/cabal,gridaphobe/cabal,Heather/cabal,ian-ross/cabal,Peaker/cabal,headprogrammingczar/cabal,corngood/cabal,plumlife/cabal,eric-stanley/cabal,lukexi/cabal,garetxe/cabal,plow-technologies/cabal