Completed the needed context to successfully test commands with Helpers

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4581 

As mentioned in symfony/symfony#12798 the explanation about the initialization is not clear.
Therefore I have added the needed context and a reference to the related cookbook article.
Well, the example in the cookbook makes use of the `Application` class from the FrameworkBundle. So it can't be moved to the components section without changes.