[Reference] Document AppKernel config settings

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

### Todo

 - [] Document the Kernel name setting

We can configure some things by overriding methods of the `Kernel` in the `AppKernel`, I think we should document those. This way, we can document how to set the charset in Symfony2.1 (see #1519).

I don't understand where the name of the Kernel is used for, so I didn't include a description for that currently. Can someone (@stof ?) tell me where this is used?