docs(pos): correct Discount.amount read behavior for 2026-07

[vctrchu]

What

Corrects the Discount.amount TSDoc on the POS cart types for the released 2026-07 API version: reading a discount from the cart returns the calculated monetary total in this field, not the value that was applied. The previous doc claimed reads return the percentage value, which is not what this version does (shop/issues-retail#33093, reported in https://community.shopify.dev/t/36265 and https://community.shopify.dev/t/36171).

Also warns that echoing a read discount back through bulkCartUpdate does not preserve it in this version, and points to 2026-10 where reads match writes (Shopify/extensibility#1657, shop/world#997169).

Older released branches have the same behavior; happy to cherry-pick this doc note to them if that's the convention.

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset directory. If the changes are user-facing and should cause a version bump, run yarn changeset to track your changes and include them in the next release CHANGELOG. If you are making simple updates to repo configuration, examples, or documentation, you do not need to add a changeset.

[github-actions]

This PR targets a stable release branch (2026-07). Once merged, the change typically also needs to be forward-ported to 2026-10-rc so it ships in the next release.

When you open the forward-port PR, include a line like this in its body so the needs-rc-port label gets removed automatically when that PR merges:

Forward-port of #4620

Accepted formats (comma-separated for multiple):

• #4620
• GH-4620
• 4620
• https://github.com/Shopify/ui-extensions/pull/4620

If a forward-port isn't needed (e.g., the change is stable-only), you can remove the needs-rc-port label manually.