docs: update 7-4-entering-bills.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a note after the "Bill Detail Fields" section explaining that all bills tables (Inbox, Approving, Paid, Archived, Scheduled) display a "Matched PO" column showing the linked purchase order number, with a tooltip link to view PO details.

Source PRs

• axolotl#4346 — feat(GREEN-1517): Add Matched PO column to Bills tables

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.