Add info in docs about behavior of download_config.num_proc
**Is your feature request related to a problem? Please describe.**

I went to override `download_config.num_proc` and was confused about what was happening under the hood. It would be nice to have the behavior documented a bit better so folks know what's happening when they use it.

**Describe the solution you'd like**

- Add note about how the default number of workers is 16. Related code:

https://github.com/huggingface/datasets/blob/7bcac0a6a0fc367cc068f184fa132b8de8dfa11d/src/datasets/download/download_manager.py#L299-L302

- Add note that if the number of workers is higher than the number of files to download, it won't use multiprocessing.

**Describe alternatives you've considered**

maybe it would also be nice to set `num_proc` = `num_files` when `num_proc` > `num_files`. 

**Additional context**

...
