Improve documentation/implementation for draw_geometries when using camera parameters

The camera parameters for [`draw_geometries`](http://www.open3d.org/docs/release/python_api/open3d.visualization.draw_geometries.html) aren't explained and appear to be non-standard, which makes them difficult to use. In computer graphics, "LookAt" is typically a view matrix that is defined by the camera's forward, up, and right vectors and its position (e.g., [[1](https://www.scratchapixel.com/lessons/mathematics-physics-for-computer-graphics/lookat-function/framing-lookat-function.html), [2](https://learnopengl.com/Getting-started/Camera)]), but here it's a vector. [In the source code](https://github.com/isl-org/Open3D/blob/a5be78cfe3da627ce6a38a7905e3e5cd6a64d811/cpp/open3d/visualization/visualizer/ViewControl.cpp#L243), the `forward` and `up` vectors appear to be standard at first, but then the `lookat` vector is added to a scaled version of the `forward` vector to set the `eye` vector, i.e., the camera's position, which is pretty odd. What I've been doing as a workaround is setting the `lookat` vector to the camera's position and then using a very small number for the `zoom` (e.g., 0.0001).