The style of the web service (Document or RPC) has absolutely nothing to do whether the service is synchronous or asynchronous. The style simply determines what the SOAP message is going to look like. You can invoke a Document style web service synchronously.
What makes your web service asynchronous? Is it only available via JMS with custom SOAP/JMS binding or do you have a different scenario or did you just think you had an async service because it was document style?