Issue #20804: Document the limitation of the unittest.mock.sentinel attributes.
diff --git a/Doc/library/unittest.mock.rst b/Doc/library/unittest.mock.rst
index ef30c01..4b9dac4 100644
--- a/Doc/library/unittest.mock.rst
+++ b/Doc/library/unittest.mock.rst
@@ -1795,6 +1795,9 @@
the same attribute will always return the same object. The objects
returned have a sensible repr so that test failure messages are readable.
+ The ``sentinel`` attributes don't preserve their identity when they are
+ :mod:`copied <copy>` or :mod:`pickled <pickle>`.
+
Sometimes when testing you need to test that a specific object is passed as an
argument to another method, or returned. It can be common to create named
sentinel objects to test this. :data:`sentinel` provides a convenient way of