Making the channel handler more useful by showing it on the prod environment

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Hi guys!

I realized there were a few practical problems with this:

1) Normally, there is no `monolog` configuration in `config.yml` - so it's odd to show an example there
2) In the `prod` environment (by default, unless you set doctrine's logging explicitly to true), the `doctrine` channel is not logged at all. So, if you tried this example in `config_prod.yml`, it probably wouldn't work. But security is always there.

Thanks!

P.S. After merging to 2.6, `container:debug` should be changed to `debug:container`.
however, when defining your services, the way to ask for a given channel is not to use ``monolog.logger.XXX`` directly. It is to inject the ``logger`` service and set the appropriate tag on your service.
this is necessary because the compiler pass reading these tags is also the place creating these services.

I'm not sure documenting the ``monolog.logger.XXX`` naming convention is actually worth it.