docs: add section on type aliases in admin + note for removing items from orders (#12996)

www/apps/api-reference/specs/admin/paths/admin_order-edits_{id}_items_item_{item_id}.yaml

@@ -2,7 +2,11 @@ post:
   operationId: PostOrderEditsIdItemsItemItem_id
   summary: Update Order Item Quantity of Order Edit
   x-sidebar-summary: Update Item Quantity
-  description: Update an existing order item's quantity of an order edit.
+  description: >
+    Update an existing order item's quantity of an order edit.
+
+    You can also use this API route to remove an item from an order by setting
+    its quantity to `0`.
   x-authenticated: true
   parameters:
     - name: id