Add info about callback in options resolver
Fix according to PR comments
feature #3437 Add info about callback in options resolver (marekkalnik)

This PR was merged into the master branch.

Discussion
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Add info about callback in options resolver

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8375)
| Applies to    | 2.5+
| Fixed tickets |

The documentation for the PR (pending) about validation callbacks in OptionsResolver.

Commits
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8231230 Fix according to PR comments
94fe8dc Add info about callback in options resolver