Added note about ProgressBar changes
feature #4206 Added note about ProgressBar changes (kbond)

This PR was merged into the master branch.

Discussion
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Added note about ProgressBar changes

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | symfony/symfony#11852
| Applies to    | 2.6
| Fixed tickets | n/a

Commits
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4e2142d Added note about ProgressBar changes