add note about hierarchy in python version

modules/imgproc/include/opencv2/imgproc.hpp

@@ -3951,6 +3951,7 @@ hierarchy[i][0] , hierarchy[i][1] , hierarchy[i][2] , and hierarchy[i][3] are se
 in contours of the next and previous contours at the same hierarchical level, the first child
 contour and the parent contour, respectively. If for the contour i there are no next, previous,
 parent, or nested contours, the corresponding elements of hierarchy[i] will be negative.
+@note In Python, hierarchy is nested inside a top level array. Use hierarchy[0][i] to access hierarchical elements of i-th contour.
 @param mode Contour retrieval mode, see #RetrievalModes
 @param method Contour approximation method, see #ContourApproximationModes
 @param offset Optional offset by which every contour point is shifted. This is useful if the