[reference] Adding the DBAL keep_slave config option

Author: weaverryan

reference/configuration/doctrine.rst

@@ -347,6 +347,8 @@ 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