[caffe2/dnnlowp] documentation for pack operator arguments (#37719)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37719
As title
Test Plan: Just updating doc
Reviewed By: hyuen
Differential Revision: D21369227
fbshipit-source-id: a45e5d0fa34aea8046eb4bb83e6c4df4d2654252
