update doc for multinomial (#17269)
Summary:
Update documentation to raise awareness of the fix in #12490. Thanks matteorr for pointing this out!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17269
Reviewed By: ezyang
Differential Revision: D14138421
Pulled By: ailzhang
fbshipit-source-id: 6433f9807a6ba1d871eba8e9d37aa6b78fa1e1fd
