Improve Tuple.Union docs

Co-authored-by: Nicolas Stucki <[email protected]>

Author: anatoliykmetyuk

library/src/scala/Tuple.scala

@@ -174,7 +174,7 @@ object Tuple {
   type Split[T <: Tuple, N <: Int] = (Take[T, N], Drop[T, N])
 
   /** Given a tuple `(T1, ..., Tn)`, returns a union of its
-   *  member types: `T1 | ... | Tn`.
+   *  member types: `T1 | ... | Tn`. Returns `Nothing` if the tuple is empty.
    */
   type Union[T <: Tuple] = Fold[T, Nothing, [x, y] =>> x | y]