minor symfony#12330 Inform about how to use new DateTime() when Clock Mocking (benjamintoussaint)

This PR was submitted for the 4.3 branch but it was merged into the 3.4 branch instead (closes symfony#12330).

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

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Commits
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8212a62 Inform about how to use new DateTime() when Clock Mocking

Author: javiereguiluz

components/phpunit_bridge.rst

@@ -331,8 +331,14 @@ Clock Mocking
 ~~~~~~~~~~~~~
 
 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()`` and ``usleep()``.
+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.
 
 To use the ``ClockMock`` class in your test, add the ``@group time-sensitive``
 annotation to its class or methods. This annotation only works when executing