num_proc=0 behave like None, num_proc=1 uses one worker (not main process) and clarify num_proc documentation
I think we can support num_proc=0 and make it equivalent to `None` to make it simpler The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7702). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. > I think we can support num_proc=0 and make it equivalent to `None` to make it simpler

Thank you @lhoestq for reviewing it. Please let me know if anything needs to be updated further.
Fixes issue #7700

This PR makes num_proc=0 behave like None in Dataset.map(), disabling multiprocessing.
It improves UX by aligning with DataLoader(num_workers=0) behavior.
The num_proc docstring is also updated to clearly explain valid values and behavior.

@SunMarc
