[package] Massage exporter docstrings (#56547)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56547
**Summary**
This commit tweaks the docstrings of `PackageExporter` so that they look
nicer on the docs website.
**Test Plan**
Continuous integration.
Test Plan: Imported from OSS
Reviewed By: ailzhang
Differential Revision: D27912965
Pulled By: SplitInfinity
fbshipit-source-id: 38c0a715365b8cfb9eecdd1b38ba525fa226a453
