Documentation for the new Uuid constraint
List the UUID constraint in the reference section
Fix formatting issues
feature #3619 [Validator] Uuid constraint reference (colinodell)

This PR was merged into the master branch.

Discussion
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[Validator] Uuid constraint reference

This adds documentation for the proposed Uuid validation constraint.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#10291)
| Applies to    | 2.5+
| Fixed tickets | N/A

Commits
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6f6c03f Fix formatting issues
3ea9137 List the UUID constraint in the reference section
f4afaab Documentation for the new Uuid constraint