Add the instancename parameter for oci8 driver

We add the instancename parameter documentation for oci8 driver

docs/en/reference/configuration.rst

@@ -193,6 +193,10 @@ pdo\_oci / oci8
    connection pooling.
 -  ``charset`` (string): The charset used when connecting to the
    database.
+-  ``instancename´´ (string): Optional parameter, complete whether to
+   add the INSTANCE_NAME parameter in the connection. It is generally used
+   to connect to an Oracle RAC server to select the name of a particular instance.   
+
 
 pdo\_sqlsrv / sqlsrv
 ^^^^^^^^^^^^^^^^^^^^