Merge pull request opencv#17742 from SoheibKadi/DetectionOutput_layer_doc

Adding comment from source code to DetectionOutputLayer class documentation

Author: alalek

modules/dnn/include/opencv2/dnn/all_layers.hpp

@@ -598,6 +598,14 @@ CV__DNN_EXPERIMENTAL_NS_BEGIN
         static Ptr<RegionLayer> create(const LayerParams& params);
     };
 
+    /**
+     * @brief Detection output layer.
+     *
+     * The layer size is: @f$ (1 \times 1 \times N \times 7) @f$
+     *    where N is [keep_top_k] parameter multiplied by batch size. Each row is:
+     *    [image_id, label, confidence, xmin, ymin, xmax, ymax]
+     *    where image_id is the index of image input in the batch.
+     */
     class CV_EXPORTS DetectionOutputLayer : public Layer
     {
     public: