Documented ExpressionLanguage extensibility

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes (symfony/symfony#12006)
| Applies to | 2.6+
| Fixed tickets | #4289
I would complete the example using the `StringExpressionLanguageProvider`:

```php
$language = new ExpressionLanguage(null, array(new StringExpressionLanguageProvider(), ...));
```

and also add a similar comment to registerProvider:

```php
// using registerProvider()
```