diff --git a/sphinx/util/typing.py b/sphinx/util/typing.py
index 4fbb592aa..fdfcb518f 100644
--- a/sphinx/util/typing.py
+++ b/sphinx/util/typing.py
@@ -186,6 +186,8 @@ def restify(cls: type | None, mode: str = 'fully-qualified-except-typing') -> st
             else:
                 return f':py:class:`{cls.__name__}`'
         elif UnionType and isinstance(cls, UnionType):
+            # Union types (PEP 585) retain their definition order when they
+            # are printed natively and ``None``-like types are kept as is.
             return ' | '.join(restify(a, mode) for a in cls.__args__)
         elif cls.__module__ in ('__builtin__', 'builtins'):
             if hasattr(cls, '__args__'):
@@ -199,6 +201,7 @@ def restify(cls: type | None, mode: str = 'fully-qualified-except-typing') -> st
         elif (inspect.isgenericalias(cls)
               and cls.__module__ == 'typing'
               and cls.__origin__ is Union):  # type: ignore[attr-defined]
+            # *cls* is defined in ``typing``, and thus ``__args__`` must exist
             return ' | '.join(restify(a, mode) for a in cls.__args__)  # type: ignore[attr-defined]
         elif inspect.isgenericalias(cls):
             if isinstance(cls.__origin__, typing._SpecialForm):  # type: ignore[attr-defined]