[torch/multiprocessing] Update documentation indicating that start_method is ignored for mp.spawn() (#33070)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33070
`start_method` parameter is intentionally ignored for `mp.spawn()`. Document this fact and point the user to `start_processes` if they want to use a different `start_method`.
Test Plan:
Warning message looks like:
```
main.py:8: UserWarning: This method only supports start_method=spawn (got: fork).
To use a different start_method use:
torch.multiprocessing.start_process(...)
warnings.warn(msg)
```
Reviewed By: ailzhang
Differential Revision: D19780235
fbshipit-source-id: 4599cd18c3ba6cc401810efe4f390290ffa8023b
