[ONNX] Update pytoch/onnx doc (#39480)
Summary:
Updated dos for operator_export_types and recently added op symbolics.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39480
Reviewed By: hl475
Differential Revision: D21877364
Pulled By: houseroad
fbshipit-source-id: 9831fe5776629da897db6d7943f830528cb916d2
