`scatter_reduce` documentation (#68580)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/63780 (part 2)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68580
Reviewed By: atalman
Differential Revision: D33800694
Pulled By: malfet
fbshipit-source-id: 2e09492a29cef115a7cca7c8209d1dcb6ae24eb9
(cherry picked from commit 696ff7594059b8b61f93475da7af7b197829061f)
