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.
This is consistent with the other helpers. But I'm not sure it makes sense. There is nowhere with this feature that you reference `DebugFormatter`. There is the class `DebugFormatterHelper` and the string you use to get the helper `debug_formatter`. So I think it makes more sense to use one of these 2 things. What do you think?