Add more information on what --threads does

[sheremet-va]

Clear and concise description of the problem

Documentation explain what threads config option does very poorly. We need to give a better description and it's difference from --runInBand in jest.

Suggested solution

• Enables isolation, if there is no --no-isolate
• If disabled, test files will run one after another

Is there anything more we could add?

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[sheremet-va]

We've actually updated docs today, I forgot we have an open issue. For anyone wondering:

This option is different from Jest's --runInBand. Vitest uses workers not only for running tests in parallel, but also to provide isolation. By disabling this option, your tests will run sequentially, but in the same global context, so you must provide isolation yourself.

This might cause all sorts of issues, if you are relying on global state (frontend frameworks usually do) or your code relies on environment to be defined separately for each test (like, @vue/test-utils). But can be a speed boost for Node tests (up to 3 times faster), that don't necessarily rely on global state or can easily bypass that.