[Form] [Hackday] [2.7] Add a deprecation note about VirtualFormAwareIterator

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12604
| License       | MIT
| Doc PR        | 
@nicolas-grekas The reason about if is because class InheritDataAwareIterator extends of this class

https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/Form/Util/InheritDataAwareIterator.php#L26