Update doc strings to make description of is_causal consistent for nn.Transformer and nn.MHA (#101089)
Summary: Update doc strings to make description of is_causal consistent for nn.Transformer and nn.MHA
Test Plan: sandcastle & github CI/CD
Differential Revision: D45737197
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101089
Approved by: https://github.com/mikaylagawarecki
