Polish DDP join API docstrings (#43973)
Summary:
Polishes DDP join api docstrings and makes a few minor cosmetic changes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43973
Reviewed By: zou3519
Differential Revision: D23467238
Pulled By: rohan-varma
fbshipit-source-id: faf0ee56585fca5cc16f6891ea88032336b3be56
