Missing documentation for torch.std and torch.var unbiased argument #2504

I've noticed that the unbiased flag is exposed to pytorch by #2043, but there's no documentation for it right now. If I didn't miss anything, altering docstring in _torch_docs.py will do it.

I would like to try to make a pull request on it, although not having any experience.
Is there anything I need to know in advance regarding making RPs on this repo and writing docstrings? (Beside those elaborated in CONTRIBUTING.md.)