Fix document around DDP uneven inputs (#57448)
Summary:
Typo fix and additional clarifications on the API.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57448
Reviewed By: SciPioneer
Differential Revision: D28153264
Pulled By: rohan-varma
fbshipit-source-id: 9bd35d918299ad7e080785d755f97b966f826615
