Document new framework option http_method_override

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#7202)
| Applies to    | master (2.3+)
| Fixed tickets | 

If ```http_method_override``` is set to ```true``` (default value), the method
[Request::enableHttpMethodParameterOverride()](http://api.symfony.com/master/Symfony/Component/HttpFoundation/Request.html#method_enableHttpMethodParameterOverride) is called before the Request is handled.

Please note that this means it is enabled by default when using the FrameworkBundle and disabled when using HttpFoundation as a standalone component (2.3).