@Override
    public String toString() {
        return
            "Buckets visited: " + bucketsVisited + "\n" +
            "Documents visited: " + documentsVisited + "\n" +
            "Bytes visited: " + bytesVisited + "\n" +
            "Documents returned: " + documentsReturned + "\n" +
            "Bytes returned: " + bytesReturned + "\n";
    }
Returns the number of documents returned to the visitor client by the backend. This number may
be lower than that returned by getDocumentsVisited() since the client-specified visitor
instance may further have filtered the set of documents returned by the backend.
vdslib/src/main/java/com/yahoo/vdslib/VisitorStatistics.java