Improved the documentation for torch.nn.functional.pad (#15984)

Improved the documentation for torch.nn.functional.pad (#15984)

Summary:
- Fixed a few typos and grammar errors.
- Changed the sentences a bit.
- Changed the format of the tuples to be consistent with padding notations in the other places. For example, `ReflectionPad2d`'s dostring contains :math:`H_{out} = H_{in} + \text{padding\_top} + \text{padding\_bottom}`.

I also made sure that the generated html doesn't break.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15984

Differential Revision: D13649939

Pulled By: soumith

fbshipit-source-id: 0abfa22a7bf1cbc6546ac4859652ce8741d41232