Clarify semantics of Data.Set.difference in Haddock (#747)

* Clarify semantics of Data.Set.difference in Haddock

in the same way done for Data.Map.difference

* Tweak haddocks formatting

Co-authored-by: Simon Jakobi <[email protected]>

Author: tomjaguarpaw

containers/src/Data/Set/Internal.hs

@@ -850,6 +850,10 @@ union t1@(Bin _ x l1 r1) t2 = case splitS x t2 of
   Difference
 --------------------------------------------------------------------}
 -- | /O(m*log(n\/m + 1)), m <= n/. Difference of two sets.
+--
+-- Return elements of the first set not existing in the second set.
+--
+-- > difference (fromList [5, 3]) (fromList [5, 7]) == singleton 3
 difference :: Ord a => Set a -> Set a -> Set a
 difference Tip _   = Tip
 difference t1 Tip  = t1