Merge pull request #23476 from fengyuentau:add_note_for_yunet

Add notes for the output format of FaceDetectorYN.detect()

Resolves https://github.com/opencv/opencv/pull/23020#issuecomment-1499010015

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/objdetect/include/opencv2/objdetect/face.hpp

@@ -54,10 +54,20 @@ public:
 
     CV_WRAP virtual int getTopK() = 0;
 
-    /** @brief A simple interface to detect face from given image
-     *
+    /** @brief Detects faces in the input image. Following is an example output.
+
+     * ![image](pics/lena-face-detection.jpg)
+
      *  @param image an image to detect
-     *  @param faces detection results stored in a cv::Mat
+     *  @param faces detection results stored in a 2D cv::Mat of shape [num_faces, 15]
+     *  - 0-1: x, y of bbox top left corner
+     *  - 2-3: width, height of bbox
+     *  - 4-5: x, y of right eye (blue point in the example image)
+     *  - 6-7: x, y of left eye (red point in the example image)
+     *  - 8-9: x, y of nose tip (green point in the example image)
+     *  - 10-11: x, y of right corner of mouth (pink point in the example image)
+     *  - 12-13: x, y of left corner of mouth (yellow point in the example image)
+     *  - 14: face score
      */
     CV_WRAP virtual int detect(InputArray image, OutputArray faces) = 0;