Added documentation about the DebugFormatter helper

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes (symfony/symfony#10627)
| Applies to | 2.6+
| Fixed tickets | #4256 

When may want to add screenshots to the article, but to get this ready quick before 2.6 is released stable, I left them out now.
The example is a bit confusing since you talk about `getHelperSet()`, but actually use `getHelper()`.