gh-156970: Set Mock.return_value's docstring (#156971)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: ByteFlowing1337

Lib/unittest/mock.py

@@ -595,7 +595,7 @@ def __set_return_value(self, value):
 
     __return_value_doc = "The value to be returned when the mock is called."
     return_value = property(__get_return_value, __set_return_value,
-                            __return_value_doc)
+                            doc=__return_value_doc)
 
 
     @property

Misc/NEWS.d/next/Library/2026-09-05-15-27-11.gh-issue-156970.DxlgbB.rst

@@ -0,0 +1 @@
+Set the docstring of :attr:`unittest.mock.Mock.return_value`.