minor symfony#8254 Improved the explanation about the doctrine caching services (javiereguiluz)

This PR was merged into the 2.7 branch. Discussion ---------- Improved the explanation about the doctrine caching services This fixes symfony#8251. Commits ------- ac46009 Improved the explanation about the doctrine caching services

Author: xabbuh

reference/configuration/doctrine.rst

@@ -299,9 +299,10 @@ certain classes, but those are for very advanced use-cases only.
 Caching Drivers
 ~~~~~~~~~~~~~~~
 
-For the caching drivers you can specify the values ``array``, ``apc``, ``apcu``,
-``memcache``, ``memcached``, ``redis``, ``wincache``, ``zenddata``, ``xcache``
-or ``service``.
+The built-in types of caching drivers are: ``array``, ``apc``, ``apcu``,
+``memcache``, ``memcached``, ``redis``, ``wincache``, ``zenddata`` and ``xcache``.
+There is a special type called ``service`` which lets you define the ID of your
+own caching service.
 
 The following example shows an overview of the caching configurations:
 
@@ -310,15 +311,17 @@ The following example shows an overview of the caching configurations:
  doctrine:
  orm:
  auto_mapping: true
+ # each caching driver type defines its own config options
  metadata_cache_driver: apc
- query_cache_driver:
- type: service
- id: my_doctrine_common_cache_service
  result_cache_driver:
  type: memcache
  host: localhost
  port: 11211
  instance_class: Memcache
+ # the 'service' type requires to define the 'id' option too
+ query_cache_driver:
+ type: service
+ id: my_doctrine_common_cache_service
 
 Mapping Configuration
 ~~~~~~~~~~~~~~~~~~~~~