[DX] New service to simplify password encoding

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | [#11306](https://github.com/symfony/symfony/pull/11306)
| Applies to    | 2.6

Add documentation for symfony/symfony PR [#11306](https://github.com/symfony/symfony/pull/11306)

New service to simplify password encoding.
Regarding the code style I would use something like this:

```php
$validPassword = $this
    ->container
    ->get('security.password_encoder')
    ->isPasswordValid($user, $plainPassword)
;
```