minor symfony#10390 add example for cache pool configuration (dbu)

This PR was submitted for the master branch but it was squashed and merged into the 3.4 branch instead (closes symfony#10390).

Discussion
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add example for cache pool configuration

the configuration reference on cache was quite unclear to me. this example should help to make it easier to understand

Commits
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8de7cdb add example for cache pool configuration

Author: javiereguiluz

reference/configuration/framework.rst

@@ -1952,6 +1952,57 @@ A list of cache pools to be created by the framework extension.
 
     For more information about how pools works, see :ref:`cache pools <component-cache-cache-pools>`.
 
+To configure a Redis cache pool with a default lifetime of 1 hour, do the following:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/packages/framework.yaml
+        framework:
+            cache:
+                pools:
+                    cache.mycache:
+                        adapter: cache.adapter.redis
+                        default_lifetime: 3600
+
+    .. code-block:: xml
+
+        <!-- config/packages/framework.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xmlns:framework="http://symfony.com/schema/dic/symfony"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                http://symfony.com/schema/dic/services/services-1.0.xsd
+                http://symfony.com/schema/dic/symfony http://symfony.com/schema/dic/symfony/symfony-1.0.xsd">
+
+            <framework:config>
+                <framework:cache>
+                    <framework:pool
+                        name="cache.mycache"
+                        adapter="cache.adapter.redis"
+                        default-lifetime=3600
+                    />
+                </framework:cache>
+                <!-- ... -->
+            </framework:config>
+        </container>
+
+    .. code-block:: php
+
+        // config/packages/framework.php
+        $container->loadFromExtension('framework', array(
+            'cache' => array(
+                'pools' => array(
+                    'cache.mycache' => array(
+                        'adapter' => 'cache.adapter.redis',
+                        'default_lifetime' => 3600,
+                    ),
+                ),
+            ),
+        ));
+
 .. _reference-cache-pools-name:
 
 name
@@ -1970,7 +2021,10 @@ adapter
 
 **type**: ``string`` **default**: ``cache.app``
 
-The name of the adapter to use. You could also use your own implementation.
+The service name of the adapter to use. You can specify one of the default
+services that follow the pattern ``cache.adapter.[type]``. Alternatively you
+can specify another cache pool as base, which will make this pool inherit the
+settings from the base pool as defaults.
 
 .. note::
 
@@ -1995,7 +2049,10 @@ provider
 
 **type**: ``string``
 
-The service name to use as provider when the specified adapter needs one.
+Overwrite the default service name or DSN respectively, if you do not want to
+use what is configured as ``default_X_provider`` under ``cache``. See the
+description of the default provider setting above for the type of adapter
+you use for information on how to specify the provider.
 
 clearer
 """""""