chore(oas): add note about returned fields in list product variants endpoint (#5360)

www/apps/api-reference/specs/admin/paths/admin_products_{id}_variants.yaml

@@ -1,9 +1,14 @@
 get:
   operationId: GetProductsProductVariants
   summary: List a Product's Variants
-  description: >-
+  description: >
     Retrieve a list of Product Variants associated with a Product. The variants
     can be paginated.
+
+
+    By default, each variant will only have the `id` and `variant_id` fields.
+    You can use the `expand` and `fields` request parameters to retrieve more
+    fields or relations.
   x-authenticated: true
   parameters:
     - in: path