[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.
I'm mixed on this. Will anyone ever really need to get the `security.encoder_factory` directly anymore? Even if you need to inject an encoding service into your own service, you could just inject the `security.password_encoder` directly. It would be easy to mock too.

So, my question to everyone is: is this worth even mentioning? Certainly, in super-advanced cases, someone really smart could find this service if they need it.