Removes beta warning from the special module documentation (#64148)
Summary:
Updates documentation per feature review. torch.special is now stable.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64148
Reviewed By: ngimel
Differential Revision: D30632049
Pulled By: mruberry
fbshipit-source-id: 8f6148ec7737e7b3a90644eeca23eb217eda513d
