cv2.findContours() no longer modifies source image (#8150)
* cv2.findContours() no longer modifies source image
Since OpenCV 3.2, cv2.findContours() no longer modifies the source image but returns a modified image as the first of three return parameters. ??? Do I have that correct ???
Also fixed up the code blocks to be valid markdown.
* ```python --> @code{.py}
Enables syntax highlighting in docs.