Modernizes add and mul documentation (#63309)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/39329.
The documentation for torch.add and torch.mul was sorely out of date and even included deprecated references. This PR modernizes their descriptions consistent with torch.sub.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63309
Reviewed By: ngimel
Differential Revision: D30338004
Pulled By: mruberry
fbshipit-source-id: ee1c2a8106af8341253cafb0003b06e8f652624d
