docs: update 2-12-custom-vs-system-fx-rates.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added "journal entries" to the list of flows where custom FX rates apply, reflecting that manual journal entry postings with multi-currency lines now respect company-level custom rates.

Source PRs

• light#7089 — feat(GREEN-1530): override-aware FX in ledger posting and FX adjustment

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.