docs: update 11-7-airwallex.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a note under "Payment status" explaining that Light monitors completed Airwallex payments for up to 7 days to detect late reversals (when the receiving bank rejects after Airwallex initially accepted), automatically reverting bills to UNPAID status when this occurs.

Source PRs

• light#6675 — feat(BLUE-610): detect and react to Airwallex late reversals

How to use this PR

1. Review the diff against the existing article — focus on accuracy and customer voice.
2. Merge if it reads well. Edit and push to this branch if it needs adjustment.
3. Close without merging if the article doesn't actually need this update.

Once we have a track record of clean drafts, we'll graduate to auto-merge with a delay buffer. For now: human-in-the-loop.