[BE] document more of FSDP checkpointing logic with a sprinkle of cleaning (#106069)
This PR should not make any functional difference. It:
- adds clearer documentation
- clarifies a type
- revises minor typos
- swaps a .keys for a .items call on a dictionary
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106069
Approved by: https://github.com/awgu
