Document Union in the stdlib

… so it shows up in help at the REPL.  My language here is substantially simpler than the description in the manual, but I believe that it is still semantically correct.

doc/stdlib/base.rst

@@ -438,6 +438,10 @@ Types
 
  Compute a type that contains the intersection of ``T`` and ``S``. Usually this will be the smallest such type or one close to it.
 
+.. function:: Union(Ts...)
+
+ Construct a special abstract type that behaves as though all of the types in ``Ts`` are its subtypes.
+
 .. function:: Val{c}
 
  Create a "value type" out of ``c``, which must be an ``isbits``