feat(fusion_shipping): UPS bill-receiver clarity + controllable commercial invoice

- Relabel UPS 'Bill My Account' -> "Bill recipient's UPS account" with clear
  help (it bills the customer's own UPS account; $0 shipping line; falls back to
  Bill Shipper when the customer has no account on file).
- Improve the customer 'UPS Account Number' field help (stored per-customer,
  auto-recalled at ship time for Bill Receiver).
- Add ups_rest_documentation_type setting (No / UPS commercial invoice) on the
  UPS REST carrier, mirroring FedEx. Default 'invoice' preserves the existing
  auto-generate-on-international behaviour; gate require_invoice on it so it can
  be turned off. Surfaced on the UPS REST config page.

Validated live on entech (UPS production): CA->US shipment generated the label
+ a 60KB commercial invoice PDF (country of origin auto = CA, HS code applied),
then voided. Bill Receiver request confirmed accepted by UPS.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_shipping/models/res_partner.py

@@ -7,5 +7,8 @@ class ResPartner(models.Model):
     property_ups_carrier_account = fields.Char(
         string="UPS Account Number",
         company_dependent=True,
-        help="UPS carrier account number for bill-my-account shipping.",
+        help="The customer's own UPS account number. Stored on the customer and "
+             "recalled automatically when you ship to them with a UPS carrier that "
+             "has \"Bill recipient's UPS account\" enabled -- UPS then bills this "
+             "account for the shipping (UPS 'Bill Receiver') instead of yours.",
     )