[ONNX] minor clarifications of docstrings (#69260) (#69549)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69549
[ONNX] minor clarifications of docstrings
1. Make description of ONNX_ATEN_FALLBACK more accurate (after #67460).
2. Specify minimum and maximum values for opset_version. This is pretty
important information and we should make users dig through source
code to find it.
Test Plan: Imported from OSS
Reviewed By: malfet
Differential Revision: D32994267
Pulled By: msaroufim
fbshipit-source-id: ba641404107baa23506d337eca742fc1fe9f0772
