message: add explanation to detectMultiScale overload functionality
diff: @@ -294,7 +294,21 @@ public:
  Size maxSize=Size() );
  /** @overload
  - if `outputRejectLevels` is `true` returns `rejectLevels` and `levelWeights`
  + This function allows you to retrieve the final stage decision certainty of classification.
  + For this, one needs to set `outputRejectLevels` on true and provide the `rejectLevels` and `levelWeights` parameter.
  + For each resulting detection, `levelWeights` will then contain the certainty of classification at the final stage.
  + This value can then be used to separate strong from weaker classifications.
  +
  + A code sample on how to use it efficiently can be found below:
  + @code
  + Mat img;
  + vector<double> weights;
  + vector<int> levels;
  + vector<Rect> detections;
  + CascadeClassifier model("/path/to/your/model.xml");
  + model.detectMultiScale(img, detections, levels, weights, 1.1, 3, 0, Size(), Size(), true);
  + cerr << "Detection " << detections[0] << " with weight " << weights[0] << endl;
  + @endcode
  */
  CV_WRAP_AS(detectMultiScale3) void detectMultiScale( InputArray image,
  CV_OUT std::vector<Rect>& objects,
  