[Docs]: Add notes on calling test.use() with worker-scoped fixtures

[vitalets]

Page(s)

https://playwright.dev/docs/next/api/class-test#test-use

Description

Add 2 notes on calling test.use with worker-scoped fixtures:

• calling test.use with worker-scoped fixture is allowed only on top-level of file, not inside any describe block
• calling test.use with worker-scoped fixture forces the whole file to be executed in a new worker

The latter one is especially important, because I was curios why my tests started to run in a new worker after I disabled screenshots:

test.use({ screenshot: 'off' });