Update libtorch docs (#21150)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/20271
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21150
Differential Revision: D15559590
Pulled By: pjh5
fbshipit-source-id: 4063bf91464425e8efe4765dc17bb7e9b7bfccc7
