docs: improve torch.matrix_exp() (#55626)
Summary:
Add a signature and make the mathematical expression related to the signature
Fixes https://github.com/pytorch/pytorch/issues/55599
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55626
Reviewed By: ngimel
Differential Revision: D27699518
Pulled By: mruberry
fbshipit-source-id: e61d76e99eb8fc36114c1c2ee90990740d78beea
