minor symfony#22139 [HttpFoundation][DX] MockArraySessionStorage: phpdocs update (MacDada)

This PR was merged into the 2.7 branch.

Discussion
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[HttpFoundation][DX] MockArraySessionStorage: phpdocs update

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Commits
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967f7a7 MockArraySessionStorage: updated phpdoc for $bags so that IDE autocompletion would work

Author: fabpot

src/Symfony/Component/HttpFoundation/Session/Storage/MockArraySessionStorage.php

@@ -58,7 +58,7 @@ class MockArraySessionStorage implements SessionStorageInterface
     protected $metadataBag;
 
     /**
-     * @var array
+     * @var array|SessionBagInterface[]
      */
     protected $bags = array();