minor symfony#21277 [DX][Cache] Set right cacheItem type hint on AdapterInterface getters (andrerom)

This PR was merged into the 3.1 branch.

Discussion
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[DX][Cache] Set right cacheItem type hint on AdapterInterface getters

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Fixes missing cache item type hint on pool getters so methods on Symfony CacheItem
is correctly suggested when using IDE's or api documentation.

As proposed here: symfony#19728 (comment)

_Note: Specifically sets array of CacheItems as return type of getItems as phpdoc and IDEs supports
this by now, and since this is specifically what is being returned. If Sami does not still support this we can adjust to what was originally suggested._

Commits
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5f7baa5 [DX][Cache] Set right type hint for cacheItem on AdapterInterface getters

Author: nicolas-grekas

src/Symfony/Component/Cache/Adapter/AdapterInterface.php

@@ -15,10 +15,23 @@
 use Symfony\Component\Cache\CacheItem;
 
 /**
- * Interface for adapters managing instances of Symfony's {@see CacheItem}.
+ * Interface for adapters managing instances of Symfony's CacheItem.
  *
  * @author Kévin Dunglas <dunglas@gmail.com>
  */
 interface AdapterInterface extends CacheItemPoolInterface
 {
+    /**
+     * {@inheritdoc}
+     *
+     * @return CacheItem
+     */
+    public function getItem($key);
+
+    /**
+     * {@inheritdoc}
+     *
+     * return \Traversable|CacheItem[]
+     */
+    public function getItems(array $keys = array());
 }