Updated Set.unions documentation to mention Foldable instead of list (#585)

The previous documentation for unions said that you need a list of sets, 
but the type says that it can be any Foldable structure:

    unions :: (Foldable f, Ord a) => f (Set a) -> Set a

Changed the wording to "The union of the sets in a Foldable structure".

Author: Morten Kolstad

Data/Set/Internal.hs

@@ -706,7 +706,7 @@ deleteMax Tip             = Tip
 {--------------------------------------------------------------------
   Union.
 --------------------------------------------------------------------}
--- | The union of a list of sets: (@'unions' == 'foldl' 'union' 'empty'@).
+-- | The union of the sets in a Foldable structure : (@'unions' == 'foldl' 'union' 'empty'@).
 unions :: (Foldable f, Ord a) => f (Set a) -> Set a
 unions = Foldable.foldl' union empty
 #if __GLASGOW_HASKELL__