Document the torch.onnx.symbolic_opset9.expand function

[svekars]

Document the torch.onnx.symbolic_opset9.expand function located in https://github.com/pytorch/pytorch/blob/main/torch/onnx/symbolic_opset9.py#L851.
Follow the Docstring Guidelines.

cc @brycebortree

[zabboud]

/assigntome