minor #24969 Replace array|\Traversable by iterable (ro0NL)

This PR was merged into the 2.7 branch.

Discussion
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Replace array|\Traversable by iterable

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #... <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!--highly recommended for new features-->

Replace `array|\Traversable` by `iterable`, favoring pure API (or less mixed at least :)) and be clear whenever phpdoc is replaced.

https://secure.php.net/manual/en/language.types.iterable.php

Commits
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278088931b Replace array|\Traversable by iterable

Author: fabpot

Validator/RecursiveContextualValidator.php

@@ -378,7 +378,7 @@ private function validateObject($object, $propertyPath, array $groups, $traversa
      * objects are iterated as well. Nested arrays are always iterated,
      * regardless of the value of $recursive.
      *
-     * @param array|\Traversable        $collection    The collection
+     * @param iterable                  $collection    The collection
      * @param string                    $propertyPath  The current property path
      * @param string[]                  $groups        The validated groups
      * @param bool                      $stopRecursion Whether to disable