Inform about how to use new DateTime() when Clock Mocking

Author: benjamintoussaint

components/phpunit_bridge.rst

@@ -412,6 +412,9 @@ The :class:`Symfony\\Bridge\\PhpUnit\\ClockMock` class provided by this bridge
 allows you to mock the PHP's built-in time functions ``time()``,
 ``microtime()``, ``sleep()``, ``usleep()`` and ``gmdate()``. Additionally the function
 ``date()`` is mocked so it uses the mocked time if no timestamp is specified.
+
+This means that you don't need to make a single change in your original code, except when using ``new DateTime()``, which must be replaced by ``DateTime::createFromFormat('U', time())`` to use the mocked ``time()`` function.
+
 Other functions with an optional timestamp parameter that defaults to ``time()``
 will still use the system time instead of the mocked time.