docs: update ForEach examples and docs

[anna-geller]

Renamed concurrency to concurrencyLimit since concurrency: 0 doesn't seem to imply that everything will run concurrently, quite the opposite — some might think that this means no concurrency at all. I feel like concurrencyLimit: 0 more clearly indicates that there are no limits in concurrency and all task groups can run concurrently.

also changed some wording and examples

@loicmathieu can you check if any tests must be adjusted for that renaming?

[loicmathieu]

LGTM, I like the notion of "task groups" which clearly describe what it is.

Tests was not updated with concurrencyLimit so they fail, I fix it, let's wait for CI before merging.