Crop point cloud json format axis_min and axis_max

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `main` branch).

### My Question

The example provided in http://www.open3d.org/docs/release/tutorial/geometry/pointcloud.html#Crop-point-cloud its is not clear how was the json generated for the example. In the example the axis_max and axis_min does not correspond to the range of the y axis of the bounding polygon or the original point cloud data. I cannot create a functional json to crop my point cloud because the args are definetly not the ymin and ymax. How was the json axis_max calculated?
"axis_max" : 4.022921085357666,
"axis_min" : -0.76341366767883301,
"bounding_polygon" :
[
[ 2.6509309513852526, 0.0, 1.6834473132326844 ],
[ 2.5786428246917148, 0.0, 1.6892074266735244 ],
[ 2.4625790337552154, 0.0, 1.6665777078297999 ],