Update docs and add new test

Author: uriyyo

Doc/library/dataclasses.rst

@@ -174,8 +174,9 @@ Module-level decorators, classes, and functions
      section.
 
    - ``slots``: If true (the default is ``False``), :attr:`__slots__` attribute
-     will be generated. If :attr:`__slots__` is defined in the class, then
-     :exc:`TypeError` is raised.
+     will be generated and new class will be returned instead of original one.
+     If :attr:`__slots__` is defined in the class, then :exc:`TypeError`
+     is raised.
 
    ``field``\s may optionally specify a default value, using normal
    Python syntax::

Lib/test/test_dataclasses.py

@@ -2823,6 +2823,16 @@ class AnotherDelivered(Base):
 
         self.assertTrue('__slots__' not in AnotherDelivered.__dict__)
 
+    def test_returns_new_class(self):
+        class A:
+            x: int
+
+        B = dataclass(A, slots=True)
+        self.assertIsNot(A, B)
+
+        self.assertFalse(hasattr(A, "__slots__"))
+        self.assertTrue(hasattr(B, "__slots__"))
+
 
 class TestDescriptors(unittest.TestCase):
     def test_set_name(self):