make broadcasting explanation clearer in matmul doc: #22763 (#45699)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45699
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D24065584
Pulled By: bdhirsh
fbshipit-source-id: 5e2cdd00ed18ad47d24d11751cfa5bee63853cc9
