doc: clarify async iterator leak

Clarifies that creating multiple async iterators from the same stream
can lead to event listener leak.

PR-URL: https://github.com/nodejs/node/pull/28997


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>