update mps note with more details (#78669)
Follow up to the comments in https://github.com/pytorch/pytorch/pull/77767#pullrequestreview-978807521
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78669
Approved by: https://github.com/kulinseth, https://github.com/anjali411
(cherry picked from commit b30b1f3decfd2b51ac2250b00a8ae7049143d855)
