feature #5409 [Reference] document new Doctrine APC cache service (xabbuh)

This PR was merged into the 2.8 branch.

Discussion
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[Reference] document new Doctrine APC cache service

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#14429)
| Applies to    | 2.8+
| Fixed tickets |

Commits
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485c8a0 document new Doctrine APC cache service

Author: weaverryan

reference/configuration/framework.rst

@@ -1372,6 +1372,13 @@ cache
 The service that is used to persist class metadata in a cache. The service
 has to implement the :class:`Symfony\\Component\\Validator\\Mapping\\Cache\\CacheInterface`.
 
+.. versionadded:: 2.8
+    The ``validator.mapping.cache.doctrine.apc`` service was introduced in
+    Symfony 2.8.
+
+Set this option to ``validator.mapping.cache.doctrine.apc`` to use the APC
+cache provide from the Doctrine project.
+
 enable_annotations
 ..................