Documented getProgress/setProgress methods
Changed the "new in Symfony 2.6" message for consistency
feature #4655 Document new progressbar methods (javiereguiluz)

This PR was merged into the 2.6 branch.

Discussion
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Document new progressbar methods

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.6+
| Fixed tickets | #4593

Commits
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6a3cbd9 Changed the "new in Symfony 2.6" message for consistency
2b86d80 Documented getProgress/setProgress methods