docs: update 6-9-customer-credits.md

[habib424]

Drafted by Ship Ship 🚢 — confidence 🟢 high

What changed

Added a brief note about tax calculation support in the "Creating a Customer Credit" section. When Avatax or Sphere tax engines are configured, line items display the tax engine name (and code for Avatax) as badges in the Tax code column, matching how invoices handle tax calculation.

Source PRs

• axolotl#4058 — feat: support other tax engines for customer credits

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.