Added feature doc for named encoders
feature #3491 Added feature doc for named encoders (tamirvs)

This PR was merged into the master branch.

Discussion
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Added feature doc for named encoders

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes, PR symfony/symfony#10005
| Applies to    | 2.5+
| Fixed tickets | -

Please check the _xml_ config example as I'm not too confident with _xml_.

Commits
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8cd63d0 Added feature doc for named encoders