[docs] Add explanation section to torch.package docs (#59833)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59833
**Summary**
This commit adds an explanation section to the `torch.package`
documentation. This section clarifies and illuminates various aspects of
the internals of `torch.package` that might be of interest to users.
**Test Plan**
Continuous integration.
Test Plan: Imported from OSS
Reviewed By: Lilyjjo
Differential Revision: D29050626
Pulled By: SplitInfinity
fbshipit-source-id: 78e0cda00f69506ef2dfc52d6df63694b502269e
