docs: Fix statement about setUpAsync() and tearDownAsync() (#6313)

Since #4732, it's wrong to says that
`setUpAsync()` and `tearDownAsync()` do nothing and can be overridden without
calling `super.setUpAsync()` and `super.tearDownAsync()`.

docs/testing.rst

@@ -369,18 +369,34 @@ functionality, the AioHTTPTestCase is provided::
 
     .. comethod:: setUpAsync()
 
-       This async method do nothing by default and can be overridden to execute
-       asynchronous code during the ``setUp`` stage of the ``TestCase``.
+       This async method can be overridden to execute asynchronous code during
+       the ``setUp`` stage of the ``TestCase``::
+
+           async def setUpAsync(self):
+               await super().setUpAsync()
+               await foo()
 
        .. versionadded:: 2.3
 
+       .. versionchanged:: 3.8
+
+          ``await super().setUpAsync()`` call is required.
+
     .. comethod:: tearDownAsync()
 
-       This async method do nothing by default and can be overridden to execute
-       asynchronous code during the ``tearDown`` stage of the ``TestCase``.
+       This async method can be overridden to execute asynchronous code during
+       the ``tearDown`` stage of the ``TestCase``::
+
+           async def tearDownAsync(self):
+               await super().tearDownAsync()
+               await foo()
 
        .. versionadded:: 2.3
 
+       .. versionchanged:: 3.8
+
+          ``await super().tearDownAsync()`` call is required.
+
     .. method:: setUp()
 
        Standard test initialization method.