Clarify Unit representation in FFI code (#223)

Author: triallax

src/Data/Unit.purs

@@ -7,6 +7,9 @@ import Data.Show (class Show)
 -- |
 -- | `Unit` is often used, wrapped in a monadic type constructor, as the
 -- | return type of a computation where only the _effects_ are important.
+-- |
+-- | When returning a value of type `Unit` from an FFI function, it is
+-- | recommended to use `undefined`, or not return a value at all.
 foreign import data Unit :: Type
 
 -- | `unit` is the sole inhabitant of the `Unit` type.