Clarify operator set reference and definition (#2721)
Clarify operator set reference and definition (#2721)

* Clarify operator sets

* Re-arrange operator set section

Co-authored-by: Ke Zhang <368059018edb346f1fd2049a2d55b57da5b85a98@microsoft.com>
onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx