minor #7111 [Doctrine] Slave/Master configuration options (Simperfit)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7111).

Discussion
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[Doctrine] Slave/Master configuration options

Like settled in the [Configuration Class](https://github.com/doctrine/DoctrineBundle/blob/master/DependencyInjection/Configuration.php#L149) options keep_slave should be a children of connections

edit : preview : http://pr-7111-scvfh6y-6qmocelev2lwe.eu.platform.sh/reference/configuration/doctrine.html

Commits
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2a38145 [Doctrine] Slave/Master configuration options

Author: xabbuh

reference/configuration/doctrine.rst

@@ -68,6 +68,8 @@ Full Default Configuration
                         profiling:            '%kernel.debug%'
                         driver_class:         ~
                         wrapper_class:        ~
+                        # the DBAL keepSlave option
+                        keep_slave:           false
                         options:
                             # an array of options
                             key:                  []
@@ -209,6 +211,7 @@ Full Default Configuration
                         logging="%kernel.debug%"
                         platform-service="MyOwnDatabasePlatformService"
                         server-version="5.6"
+                        keep-slave="false"
                     >
                         <doctrine:option key="foo">bar</doctrine:option>
                         <doctrine:mapping-type name="enum">string</doctrine:mapping-type>
@@ -405,8 +408,6 @@ The following block shows all possible configuration keys:
                     enum: string
                 types:
                     custom: Acme\HelloBundle\MyCustomType
-                # the DBAL keepSlave option
-                keep_slave:           false
 
     .. code-block:: xml