bpo-38753: AsyncMock added in version 3.8 (GH-17102)

diff --git a/Doc/library/unittest.mock.rst b/Doc/library/unittest.mock.rst
index 746cf76..7faecff 100644
--- a/Doc/library/unittest.mock.rst
+++ b/Doc/library/unittest.mock.rst
@@ -915,6 +915,7 @@
   >>> mock.async_foo
   <AsyncMock name='mock.async_foo' id='...'>
 
+  .. versionadded:: 3.8
 
   .. method:: assert_awaited()