doc: improve parametrizing conditional raising example to also check for values

What one typically actually wants in such a case is both, checking for some
resulting values *and* checking for some expected exception.

Since this is easily possible with the `nullcontext` context manager, adapt the
example accordingly.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

doc/en/example/parametrize.rst

@@ -657,7 +657,10 @@ Use :func:`pytest.raises` with the
 :ref:`pytest.mark.parametrize ref` decorator to write parametrized tests
 in which some tests raise exceptions and others do not.
 
-It may be helpful to use ``nullcontext`` as a complement to ``raises``.
+``contextlib.nullcontext`` can be used to test cases that are not expected to
+raise exceptions but that should result in some value. The value is given as the
+``enter_result`` parameter, which will be available as the ``with`` statement’s
+target (``e`` in the example below).
 
 For example:
 
@@ -671,16 +674,16 @@ For example:
     @pytest.mark.parametrize(
         "example_input,expectation",
         [
-            (3, nullcontext()),
+            (3, nullcontext(2)),
-            (2, nullcontext()),
+            (2, nullcontext(3)),
-            (1, nullcontext()),
+            (1, nullcontext(6)),
             (0, pytest.raises(ZeroDivisionError)),
         ],
     )
     def test_division(example_input, expectation):
         """Test how much I know division."""
-        with expectation:
+        with expectation as e:
-            assert (6 / example_input) is not None
+            assert (6 / example_input) == e
 
 In the example above, the first three test cases should run unexceptionally,
 while the fourth should raise ``ZeroDivisionError``.