Enhance the documentation for DistributedDataParallel from torch.nn.parallel.distribu...

Enhance the documentation for DistributedDataParallel from torch.nn.parallel.distributed (#16010)

Summary:
- a typo fixed
- made the docs consistent with #5108

And maybe one more change is needed. According to the current docs
> The batch size should be larger than the number of GPUs used **locally**.

But shouldn't the batch size be larger than the number of GPUs used **either locally or remotely**? Sadly, I couldn't experiment this with my single GPU.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16010

Differential Revision: D13709516

Pulled By: ezyang

fbshipit-source-id: e44459a602a8a834fd365fe46e4063e9e045d5ce