[docs][1.5] update RPC docs to reflect correct use of dist_autograd backwards and dist_optim step() (#34670)
Summary:
- Clarify that `torch.distributed.autograd.backwards()` does not use the current thread local autograd context, instead it looks it up based on the context_id passed in
- Clarify the same for `torch.distributeed.optimizer.optim.step()`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34670
Differential Revision: D20427645
Pulled By: rohan-varma
fbshipit-source-id: a1a88de346cdd4dbe65fb2b7627157f86fd2b6a3
