#13387: add note about checking the exact type in assertIsInstance doc.

Doc/library/unittest.rst

@@ -895,6 +895,8 @@ Test cases
 
       Test that *obj* is (or is not) an instance of *cls* (which can be a
       class or a tuple of classes, as supported by :func:`isinstance`).
+      To check for a specific type (without including superclasses) use
+      :func:`assertIs(type(obj), cls) <assertIs>`.
 
       .. versionadded:: 2.7