[Doctrine] Document database replicas

Author: phansys

reference/configuration/doctrine.rst

@@ -52,6 +52,10 @@ The following block shows all possible configuration keys:
                 path:                 '%kernel.project_dir%/var/data/data.sqlite'
                 memory:               true
                 unix_socket:          /tmp/mysql.sock
+                replicas:
+                    read_only_replica:
+                        url: mysql://db_user:[email protected]:3306/db_name_replica
+                keep_replica: true
                 # the DBAL wrapperClass option
                 wrapper_class:        App\DBAL\MyConnectionWrapper
                 charset:              utf8mb4