chore: Describe current aclose behavior in async redis store

[jsonbailey]

Rewords a comment in the async Redis big segment store to describe current behavior instead of narrating version history.

Before: # aclose() replaced the async close() in redis-py 5.0.1; the [redis] extra allows >= 4.2
After: # Prefer aclose() (redis-py 5.0.1+); older supported versions (>= 4.2) only have close().

Comment-only; no behavior change. Part of the async comment-style cleanup.

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Note

Low Risk
Documentation-only comment change with no executable code or dependency changes.

Overview
Comment-only update in _AsyncRedisBigSegmentStore.stop(): the note above the aclose() / close() branch is reworded to describe current behavior (prefer aclose() on redis-py 5.0.1+, fall back to close() on older supported redis-py ≥ 4.2) instead of narrating that aclose() replaced close() in 5.0.1.

No logic, dependencies, or runtime behavior changes.

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