Add docstring type formatting guidelines to `CONTRIBUTING.md` (#83536)
This PR builds on the following past PRs, and serves to help improve the consistency of PyTorch's docstring formatting.
* `boolean` -> `bool` and `string` -> `str`: https://github.com/pytorch/pytorch/pull/82410
* Don't use plural of types: https://github.com/pytorch/pytorch/pull/82474
* Capitalize the Callable type, `callable` -> `Callable` : https://github.com/pytorch/pytorch/pull/82487
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83536
Approved by: https://github.com/H-Huang, https://github.com/albanD
