Added documentation about the DebugFormatter helper
Applied comments from our great reviewers
Reordered list of helpers
feature #4485 Added documentation about the DebugFormatter helper (WouterJ)

This PR was merged into the 2.6 branch.

Discussion
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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.

Commits
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99f31fc Reordered list of helpers
6a02f68 Applied comments from our great reviewers
363e38f Added documentation about the DebugFormatter helper