Doc: save_to_disk, `num_proc` will affect `num_shards`, but it's not documented
### Describe the bug

[`num_proc`](https://huggingface.co/docs/datasets/main/en/package_reference/main_classes#datasets.DatasetDict.save_to_disk.num_proc) will affect `num_shards`, but it's not documented


### Steps to reproduce the bug

Nothing to reproduce

### Expected behavior

[document of `num_shards`](https://huggingface.co/docs/datasets/main/en/package_reference/main_classes#datasets.DatasetDict.save_to_disk.num_shards) explicitly says that it depends on `max_shard_size`, it should also mention `num_proc`.


### Environment info

datasets main document
I agree this should be documented