[ONNX] minor doc improvements and cleanup (#62514) (#64373)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64373
* Fix some bad formatting and clarify things in onnx.rst.
* In `export_to_pretty_string`:
* Add documentation for previously undocumented args.
* Document that `f` arg is ignored and mark it deprecated.
* Update tests to stop setting `f`.
* Warn if `_retain_param_name` is set.
* Use double quotes for string literals in test_operators.py.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D30905271
Pulled By: malfet
fbshipit-source-id: 3627eeabf40b9516c4a83cfab424ce537b36e4b3
