Changed convexHull's documentation to essentially invert the meaning of clockwise

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.