gh-90015: Document that PEP-604 unions do not support forward references (#105366)

Author: AlexWaygood

Doc/library/stdtypes.rst

@@ -5162,6 +5162,14 @@ enables cleaner type hinting syntax compared to :data:`typing.Union`.
       def square(number: int | float) -> int | float:
           return number ** 2
 
+   .. note::
+
+      The ``|`` operand cannot be used at runtime to define unions where one or
+      more members is a forward reference. For example, ``int | "Foo"``, where
+      ``"Foo"`` is a reference to a class not yet defined, will fail at
+      runtime. For unions which include forward references, present the
+      whole expression as a string, e.g. ``"int | Foo"``.
+
 .. describe:: union_object == other
 
    Union objects can be tested for equality with other union objects.  Details: