Clarify topk doc (#65938)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/50331
<img width="855" alt="Screen Shot 2021-10-01 at 11 23 23 AM" src="https://user-images.githubusercontent.com/17888388/136036611-f2bd9c77-61b4-4ab8-85eb-44f50c1e03d7.png">
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65938
Reviewed By: bdhirsh
Differential Revision: D31314875
Pulled By: samdow
fbshipit-source-id: bdd9425fd748710f8a64ed1989e1938dd358780f
