Add documentation to Dispatch.h (#20339)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20339
ghimport-source-id: 99b6cf9f03be8d55674a43c8a7e19a42b75143f1
Differential Revision: D15295652
Pulled By: ezyang
fbshipit-source-id: f3b127ed3f53f13931596c06e181056940443290
