minor symfony#18168 FrameworkBundle: Client: getContainer(): fixed phpdoc (MacDada)

This PR was merged into the 2.3 branch.

Discussion
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FrameworkBundle: Client: getContainer(): fixed phpdoc

| Q             | A
| ------------- | ---
| Branch        | lowest applicable and maintained version
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

The kernel [might be shut down](https://github.com/symfony/symfony/blob/2.3/src/Symfony/Component/HttpKernel/Kernel.php#L164) and then the method will return null instead of a ContainerInterface object.

---

I've stumbled upon when I was trying to do this in my [test](https://github.com/symfony/framework-bundle/blob/master/Test/WebTestCase.php):

```php
	/**
	 * @var Client|null
	 */
	private static $client;

	public static function tearDownAfterClass()
	{
		$entityManager = $client->getContainer()
			->get('doctrine')
			->getManager();
		// remove entities created by tests
	}

	public function testWhatever()
	{
		self::$client = self::createClient();
		// entities are created
	}
```

`$client->getContainer()` has given me `null` while I was expecting the container, as the PHPDoc say I would. Unpleasant debugging and WTFing.

Commits
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7462fa5 FrameworkBundle: Client: getContainer(): fixed phpdoc

Author: nicolas-grekas

src/Symfony/Bundle/FrameworkBundle/Client.php

@@ -41,7 +41,7 @@ public function __construct(KernelInterface $kernel, array $server = array(), Hi
     /**
      * Returns the container.
      *
-     * @return ContainerInterface
+     * @return ContainerInterface|null Returns null when the Kernel has been shutdown or not started yet
      */
     public function getContainer()
     {