[3.12] gh-65453: Docs - clarify AttributeError behaviour on PropertyMock (GH-121666) (GH-121969)

Fixed at EuroPython 24 sprints.
(cherry picked from commit 94e6644)

Co-authored-by: Vlastimil Zíma <[email protected]>

Author: miss-islington

Doc/library/unittest.mock.rst

@@ -856,6 +856,20 @@ object::
     3
     >>> p.assert_called_once_with()
 
+.. caution::
+
+    If an :exc:`AttributeError` is raised by :class:`PropertyMock`,
+    it will be interpreted as a missing descriptor and
+    :meth:`~object.__getattr__` will be called on the parent mock::
+
+        >>> m = MagicMock()
+        >>> no_attribute = PropertyMock(side_effect=AttributeError)
+        >>> type(m).my_property = no_attribute
+        >>> m.my_property
+        <MagicMock name='mock.my_property' id='140165240345424'>
+
+    See :meth:`~object.__getattr__` for details.
+
 
 .. class:: AsyncMock(spec=None, side_effect=None, return_value=DEFAULT, wraps=None, name=None, spec_set=None, unsafe=False, **kwargs)