The orientation of convexHull's result is currently the opposite of what the
documentation would suggest:
>>> import cv2, numpy as np
>>> points = np.array([[0,0],[0,1],[1,0]], dtype=np.int32)
>>> cv2.convexHull(points, clockwise=False)
array([[[1, 0]],
[[0, 1]],
[[0, 0]]], dtype=int32)
>>> cv2.convexHull(points, clockwise=True)
array([[[0, 0]],
[[0, 1]],
[[1, 0]]], dtype=int32)
Changing the function itself is probably not a good idea at this point, so
this fixes the documentation by flipping the coordinate system.
I also removed the mention of the origin, since it's irrelevant for this
function.