Added docs for ProxyAdapter

components/cache/cache_pools.rst

@@ -148,7 +148,21 @@ lifetime applied to those cache items (by default it's ``0``).
 Proxy Cache Adapter
 ~~~~~~~~~~~~~~~~~~~
 
-.. TODO: what is this adapter useful for?
+This adapter is useful to integrate in your application cache pools not created
+with the Symfony Cache component. As long as those cache pools implement the
+``CacheItemPoolInterface`` interface, this adapter allows you to get items from
+that external cache and save them in the Symfony cache of your application::
+
+ use Symfony\Component\Cache\Adapter\ProxyAdapter;
+
+ // ... create $nonSymfonyCache somehow
+ $cache = new ProxyAdapter($nonSymfonyCache);
+
+The adapter accepts two additional optional arguments: the namespace (``''`` by
+default) and the default lifetime (``0`` by default).
+
+Another use case for this adapter is to get statistics and metrics about the
+cache hits and misses thanks to the ``getHits()`` and ``getMisses()`` methods.
 
 Doctrine Cache Adapter
 ~~~~~~~~~~~~~~~~~~~~~~