What is the difference between using the service defined in /umbraco/webservice.asmx vs. the lower level ones (ie, those defined in /umbraco/webservices/api/ ?

If the answer to the above doesn't rule using it out, can someone link or describe the capabilities of the getDocumentsBySearch service method?  What is the expected format of the search string? 

I'm having alot of difficulty finding any examples using the webservice api.  Am I overlooking it, or is that just a pain point right now?

Thanks!