Add more examples

Based on very valid observation here
(https://groups.google.com/forum/#!topic/elm-discuss/_vvE0OWNKDU) that
these docs were bad before.

libraries/Maybe.elm

@@ -21,27 +21,45 @@ numbers).
 -}
 data Maybe a = Just a | Nothing
 
-{-| Apply a function to the contents of a `Maybe`.
+{-| Provide a default value and a function to extract the contents of a `Maybe`.
-Return default when given `Nothing`.
+When given `Nothing` you get the default, when given a `Just` you apply the
+function to the associated value.
+
+      isPositive : Maybe Int -> Bool
+      isPositive maybeInt = maybe False (\n -> n > 0) maybeInt
+
+      map : (a -> b) -> Maybe a -> Maybe b
+      map f m = maybe Nothing (\x -> Just (f x)) m
 -}
 maybe : b -> (a -> b) -> Maybe a -> b
 maybe b f m = case m of
                 Just v  -> f v
                 Nothing -> b
 
-{-| Check if constructed with `Just`. 
+{-| Check if a maybe happens to be a `Just`.
+
+      isJust (Just 42) == True
+      isJust (Just []) == True
+      isJust Nothing   == False
 -}
 isJust : Maybe a -> Bool
 isJust = maybe False (\_ -> True)
 
 {-| Check if constructed with `Nothing`.
+
+      isNothing (Just 42) == False
+      isNothing (Just []) == False
+      isNothing Nothing   == True
 -}
 isNothing : Maybe a -> Bool
 isNothing = not . isJust
 
+cons : Maybe a -> [a] -> [a]
 cons mx xs = maybe xs (\x -> x :: xs) mx
 
 {-| Filters out Nothings and extracts the remaining values.
+
+      justs [Just 0, Nothing, Just 5, Just 7] == [0,5,7]
 -}
 justs : [Maybe a] -> [a]
 justs = foldr cons []