You can use some tool like the free SoapUI to generate example requests and responses from wsdl. Just install SoapUI, create a new SOAP project with the wsdl from Salesforce, add the generated request to a Mock Service, and let SoapUI generate a Mock Response. There's your example, which should look something like this, with the ? replaced with true/false:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:out="http://soap.sforce.com/2005/09/outbound">
<soapenv:Header/>
<soapenv:Body>
<out:notificationsResponse>
<out:Ack>?</out:Ack>
</out:notificationsResponse>
</soapenv:Body>
</soapenv:Envelope>