Clarify add new op documentation (#4150)
Clarify add new op documentation (#4150)

Signed-off-by: Ganesan Ramalingam <2c03bc4d5fb01d05f9070ed6a9a29a6918c04271@microsoft.com>
onnx/onnx,onnx/onnx,onnx/onnx,onnx/onnx