bpo-42391: Clarify documentation of TestCase.assertIs (GH-23348)

Removing 'evaluate' makes it more consistent with other assertX entries.
diff --git a/Doc/library/unittest.rst b/Doc/library/unittest.rst
index 51e1011..0a09935 100644
--- a/Doc/library/unittest.rst
+++ b/Doc/library/unittest.rst
@@ -897,8 +897,7 @@
    .. method:: assertIs(first, second, msg=None)
                assertIsNot(first, second, msg=None)
 
-      Test that *first* and *second* evaluate (or don't evaluate) to the
-      same object.
+      Test that *first* and *second* are (or are not) the same object.
 
       .. versionadded:: 3.1