Enhance DDP docstrings for DDP + RPC support. (#39916)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39916
ghstack-source-id: 105999275
Test Plan: waitforbuildbot
Differential Revision: D22013190
fbshipit-source-id: be3bb12b2281579610581b809c822ab6b027fa71
