CLI Option `processes`, what is a good value?

### Description

The documentation contains very little information about the effect of the `--processes` parameter. It mentions the default value, but not much more.

I am struggling to understand how many processes would be a sensible value to improve the speed of the scan? The default is 1, I guess that is a conservative value to ensure Scancode-Toolkit works even in resource restricted environments (e.g. low powered VM).

Should the processes be equal to the number of cores of the CPU? Is there an amount of processes which is just too much? Because not every part of the code can be parallelized.

While it is difficult to give a general recommendation. It would be beneficial to have more information, so that users can find there individual "good" value.

### Link to Documentation Page

https://scancode-toolkit.readthedocs.io/en/stable/cli-reference/list-options.html#all-core-scan-options

### Select Category

- [ ] Inconsistency
- [ ] New Section Request
- [x] General Improvement
- [ ] Typo/Mistakes
- [ ] Other

For more information have a look at: #608