Update key_padding_mask arg docs in MHA module (#39321)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/39321
Reviewed By: zhangguanheng66
Differential Revision: D21825488
Pulled By: Nayef211
fbshipit-source-id: 41ee09e683c4ae838cfd488a342088d591e806e4
