[DDP] Enhance comm hook docs (#61677)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61677
Specify return type more clearly, 2) Misc fixes
ghstack-source-id: 133657895
Test Plan: CI
Reviewed By: zhaojuanmao
Differential Revision: D29701384
fbshipit-source-id: 7f77b99065bd2977153f397745e07b75bbdd7a94
