bpo-41877: Improve docs for assert misspellings check in mock (GH-23729)



This is a follow-up to
https://github.com/python/cpython/commit/4662fa9bfe4a849fe87bfb321d8ef0956c89a772.
That original commit expanded guards against misspelling assertions on
mocks. This follow-up updates the documentation and improves the error
message by pointing out the potential cause and solution.

Automerge-Triggered-By: GH:gpshead
diff --git a/Lib/unittest/test/testmock/testmock.py b/Lib/unittest/test/testmock/testmock.py
index dfcf1ef..016905c 100644
--- a/Lib/unittest/test/testmock/testmock.py
+++ b/Lib/unittest/test/testmock/testmock.py
@@ -1598,7 +1598,7 @@
     #Issue21238
     def test_mock_unsafe(self):
         m = Mock()
-        msg = "Attributes cannot start with 'assert' or its misspellings"
+        msg = "is not a valid assertion. Use a spec for the mock"
         with self.assertRaisesRegex(AttributeError, msg):
             m.assert_foo_call()
         with self.assertRaisesRegex(AttributeError, msg):