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`.
why didn't you add a comment for this line?