Improve the basic example.

* Show both the decorator and regular form for assertRaises()
* Use assertTrue() instead of assertIn() to teach useful minimal subset of the API

Doc/library/unittest.rst

@@ -116,14 +116,18 @@ Here is a short script to test three functions from the :mod:`random` module::
            self.seq.sort()
            self.assertEqual(self.seq, range(10))
 
+           # should raise an exception for an immutable sequence
+           self.assertRaises(TypeError, random.shuffle, (1,2,3))
+
        def test_choice(self):
            element = random.choice(self.seq)
-           self.assertIn(element, self.seq)
+           self.assertTrue(element in self.seq)
 
        def test_sample(self):
-           self.assertRaises(ValueError, random.sample, self.seq, 20)
+           with self.assertRaises(ValueError):
+               random.sample(self.seq, 20)
            for element in random.sample(self.seq, 5):
-               self.assertIn(element, self.seq)
+               self.assertTrue(element in self.seq)
 
    if __name__ == '__main__':
        unittest.main()