clarify the possible class/interface of the cache

When the fallback cache pool is returned (on PHP 5.6, HHVM, or when
Opcache is disabled), the configured service can be any implementation
of the CacheItemPoolInterface.

src/Symfony/Bundle/FrameworkBundle/Resources/config/serializer.xml

@@ -82,7 +82,7 @@
             <tag name="kernel.cache_warmer" />
         </service>
 
-        <service id="serializer.mapping.cache.symfony" class="Symfony\Component\Cache\Adapter\PhpArrayAdapter">
+        <service id="serializer.mapping.cache.symfony" class="Psr\Cache\CacheItemPoolInterface">
             <factory class="Symfony\Component\Cache\Adapter\PhpArrayAdapter" method="create" />
             <argument>%serializer.mapping.cache.file%</argument>
             <argument type="service" id="cache.serializer" />