Add examples for alt (#58)

src/Control/Alt.purs

@@ -18,6 +18,21 @@ import Data.Semigroup (append)
 -- |
 -- | For example, the `Array` (`[]`) type is an instance of `Alt`, where
 -- | `(<|>)` is defined to be concatenation.
+-- |
+-- | A common use case is to select the first "valid" item, or, if all items
+-- | are "invalid", the last "invalid" item.
+-- |
+-- | For example:
+-- |
+-- | ```purescript
+-- | import Control.Alt ((<|>))
+-- | import Data.Maybe (Maybe(..)
+-- | import Data.Either (Either(..))
+-- |
+-- | Nothing <|> Just 1 <|> Just 2 == Just 1
+-- | Left "err" <|> Right 1 <|> Right 2 == Right 1
+-- | Left "err 1" <|> Left "err 2" <|> Left "err 3" == Left "err 3"
+-- | ```
 class Functor f <= Alt f where
   alt :: forall a. f a -> f a -> f a