update `unsafe` flag documentation (#60899)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60899
modify documentation for `unsafe` flag in `parametrize.py`
ghstack-source-id: 132591862
Test Plan:
shouldn't modify code behavior but as a double check,
`buck test mode/dev-nosan //caffe2/test:nn -- --exact 'caffe2/test:nn - test_register_and_remove_parametrization (test_nn.TestNN)'`
https://pxl.cl/1L1fw
Reviewed By: albanD
Differential Revision: D29436688
fbshipit-source-id: 85499ad22b49ad992507b9ed5e7def8231cbfeba
