"extent" documentation is incomplete and unclear

Feedback from a user:

> The problem is that the `extent` parameter is used in points3d, axes, and outline, but it does different things. When used in points3d it just rescales the coordinates so when axes is called (w/o the extent parameter) it adds axes to these rescaled coordinates. On the other hand, if you use points3d w/o extent and then call axes with extent if will draw axes according to extent. So for example if the extent of my data is [-10,10,-10,10,-10,10] and I just call points3d w/o extent and then then call axes with `extent=[-100,100,-100,100,-100,100]` then I will have the axes around a cube with 200 sides, and my points will be a tiny blob in the center.