fix docs for new 3.0 behavior

Author: milkie

source/reference/method/rs.addArb.txt

@@ -14,8 +14,3 @@ Description
    The :method:`rs.addArb()` method takes the following parameter:
 
    .. include:: /includes/apiargs/method-rs.addArb-param.rst
-
-   This function briefly disconnects the shell and forces a reconnection
-   as the replica set renegotiates which member will be :term:`primary`.
-   As a result, the shell displays an error even if this command
-   succeeds.