1. documentation for depends / produces elements in wsimport Ant task
official documentation additional documentation
2. purpose of <soapAction> attribute in WSDL
In the WSDL omitting the soapAction attribute is equivalent to setting soapAction="". It means that the SOAPAction HTTP header will be empty. The WS-I Basic Profile requires that a SOAP envelope inside an HTTP POST request is accompanied by an HTTP SOAPAction header - however it permits the SOAPAction header to be empty. The soapAction attribute simply specifies the content of the SOAPAction HTTP header. The content of the SOAPAction HTTP header is supposed to communicate the "intent" (e.g. operation) of the SOAP envelope, so that proxies and firewalls can act on it without looking into the XML SOAP envelope.
3. namespaces for WSDL versions
• WSDL 1.1 → http://schemas.xmlsoap.org/wsdl/ (source)
• WSDL 1.2 → http://www.w3.org/2002/07/wsdl (source)
4. namespaces for SOAP versions
• SOAP 1.2 → http://www.w3.org/2003/05/soap-envelope (source)
• SOAP 1.1 → http://schemas.xmlsoap.org/soap/envelope/ (source)
• SOAP 1.1 → confusingly, there appears to also be another namespace, but that's probably for an WSDL 1.1 binding to SOAP 1.1: http://schemas.xmlsoap.org/wsdl/soap/
5. actual examples of SOAP envelopes for SOAP 1.1 and SOAP 1.2

SOAP 1.1

The following is a sample SOAP 1.1 request and response. The placeholders shown need to be replaced with actual values.

POST /vor10/ristandardservice.asmx HTTP/1.1
Host: nvo.stsci.edu
Content-Type: text/xml; charset=utf-8
Content-Length: length
SOAPAction: "http://www.ivoa.net/wsdl/RegistrySearch/v1.0#KeywordSearch"

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <KeywordSearch xmlns="http://www.ivoa.net/wsdl/RegistrySearch/v1.0">
      <keywords xmlns="">string</keywords>
      <orValues xmlns="">boolean</orValues>
      <from xmlns="">positiveInteger</from>
      <max xmlns="">positiveInteger</max>
      <identifiersOnly xmlns="">boolean</identifiersOnly>
    </KeywordSearch>
  </soap:Body>
</soap:Envelope>

HTTP/1.1 200 OK
Content-Type: text/xml; charset=utf-8
Content-Length: length

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <SearchResponse xmlns="http://www.ivoa.net/wsdl/RegistrySearch/v1.0">
      <VOResources from="positiveInteger" numberReturned="positiveInteger" more="boolean" xmlns="http://www.ivoa.net/xml/RegistryInterface/v1.0" />
    </SearchResponse>
  </soap:Body>
</soap:Envelope>

SOAP 1.2

The following is a sample SOAP 1.2 request and response. The placeholders shown need to be replaced with actual values.

POST /vor10/ristandardservice.asmx HTTP/1.1
Host: nvo.stsci.edu
Content-Type: application/soap+xml; charset=utf-8
Content-Length: length

<?xml version="1.0" encoding="utf-8"?>
<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
  <soap12:Body>
    <KeywordSearch xmlns="http://www.ivoa.net/wsdl/RegistrySearch/v1.0">
      <keywords xmlns="">string</keywords>
      <orValues xmlns="">boolean</orValues>
      <from xmlns="">positiveInteger</from>
      <max xmlns="">positiveInteger</max>
      <identifiersOnly xmlns="">boolean</identifiersOnly>
    </KeywordSearch>
  </soap12:Body>
</soap12:Envelope>

HTTP/1.1 200 OK
Content-Type: application/soap+xml; charset=utf-8
Content-Length: length
<?xml version="1.0" encoding="utf-8"?>
<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
  <soap12:Body>
    <SearchResponse xmlns="http://www.ivoa.net/wsdl/RegistrySearch/v1.0">
      <VOResources from="positiveInteger" numberReturned="positiveInteger" more="boolean" xmlns="http://www.ivoa.net/xml/RegistryInterface/v1.0" />
    </SearchResponse>
  </soap12:Body>
</soap12:Envelope>