Add notes for the output format of FaceDetectorYN.detect()
Resolves https://github.com/opencv/opencv/pull/23020#issuecomment-1499010015
### Pull Request Readiness Checklist
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- [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
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- [x] There is a reference to the original bug report and related work
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Patch to opencv_extra has the same branch name.
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