Use to work on 3.1. Here is the quote from the doc - https://developer.atlassian.com/display/CONFDEV/Confluence+XML-RPC+and+SOAP+APIs

String exportSite(String token, boolean exportAttachments) - exports a Confluence instance and returns a String holding the URL for the download. The boolean argument indicates whether or not attachments ought to be included in the export.

On 3.5.13 it returns a string something like:

The backup was successfully created at /Applications/atlassian/confluence-3.5.13/home/temp/xmlexport-20120108-211601-5.zip. This file will be deleted in 24 hours.

Related issue: https://studio.plugins.atlassian.com/browse/CSOAP-125