bpo-28911: Clarify the behaviour of assert_called_once_with. (#252)

(cherry picked from commit 9d56b34af2efc4e266bf3ae62da5cd2e422a42be)
diff --git a/Lib/unittest/mock.py b/Lib/unittest/mock.py
index b6b3836..5f97728 100644
--- a/Lib/unittest/mock.py
+++ b/Lib/unittest/mock.py
@@ -815,8 +815,8 @@
 
 
     def assert_called_once_with(_mock_self, *args, **kwargs):
-        """assert that the mock was called exactly once and with the specified
-        arguments."""
+        """assert that the mock was called exactly once and that that call was
+        with the specified arguments."""
         self = _mock_self
         if not self.call_count == 1:
             msg = ("Expected '%s' to be called once. Called %s times." %