[Cache] Document "framework.cache.directory" option

Author: geoffrey-brier

reference/configuration/framework.rst

@@ -102,6 +102,8 @@ Configuration
     * :ref:`cache <reference-serializer-cache>`
     * :ref:`enable_annotations <reference-serializer-enable_annotations>`
     * :ref:`name_converter <reference-serializer-name_converter>`
+* `cache`_
+    * `directory`_
 
 secret
 ~~~~~~
@@ -1446,6 +1448,19 @@ value.
     For more information, see
     :ref:`component-serializer-converting-property-names-when-serializing-and-deserializing`.
 
+cache
+~~~~~
+
+directory
+.........
+
+.. versionadded:: 3.1
+The ``directory`` option was introduced in Symfony 3.1.
+
+**type**: ``string`` **default**: ``%kernel.cache_dir%/pools``
+
+The path to the cache directory used by the filesystem cache adapter.
+
 Full Default Configuration
 --------------------------
 
@@ -1580,6 +1595,10 @@ Full Default Configuration
                 file_cache_dir:       '%kernel.cache_dir%/annotations'
                 debug:                '%kernel.debug%'
 
+            # cache configuration
+            cache:
+                directory: '%kernel.cache_dir%/pools'
+
 .. _`HTTP Host header attacks`: http://www.skeletonscribe.net/2013/05/practical-http-host-header-attacks.html
 .. _`Security Advisory Blog post`: https://symfony.com/blog/security-releases-symfony-2-0-24-2-1-12-2-2-5-and-2-3-3-released#cve-2013-4752-request-gethost-poisoning
 .. _`Doctrine Cache`: http://docs.doctrine-project.org/projects/doctrine-common/en/latest/reference/caching.html