doc: clarify forEach concurrency and ordering

[mmustafasenoglu]

Adds a short note to the readable.forEach docs for #66106, explaining that with concurrency greater than 1 the fn calls still complete in chunk order, so a slow call on an early chunk holds up the later ones.

Fixes: #66106

Disclosure: I used an AI coding assistant to help draft this docs change. I personally verified the wording against the implementation in lib/internal/streams/operators.js (forEach delegates to map, which awaits the queued calls in order) and confirmed the behavior with a local script.

[github-actions]

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