[Cookbook][External Parameters] Enhance content

Enhance description of environment variables used as container parameters

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | -
I would reword this to something like:

> In debug mode, this is required since only a change to a configuration file that is loaded by Symfony triggers your configuration to be re-evaluated.