Update parallel_for/reduce doc (#28545)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28545
* **#28545 Update parallel_for/reduce doc**
Test Plan: Imported from OSS
Differential Revision: D18251534
Pulled By: albanD
fbshipit-source-id: e743e4acfe1a4b5a329c11f7d03efd34d19efda8
