Merge pull request #140 from square/eden/update-optional-tender-id

Update docs to reflect optional `tender_id`.

doc/models/refund.md

@@ -14,7 +14,7 @@ Represents a refund processed for a Square transaction.
 | `id` | `String` | Required | The refund's unique ID.<br>**Constraints**: *Maximum Length*: `255` |
 | `location_id` | `String` | Required | The ID of the refund's associated location.<br>**Constraints**: *Maximum Length*: `50` |
 | `transaction_id` | `String` | Optional | The ID of the transaction that the refunded tender is part of.<br>**Constraints**: *Maximum Length*: `192` |
-| `tender_id` | `String` | Required | The ID of the refunded tender.<br>**Constraints**: *Maximum Length*: `192` |
+| `tender_id` | `String` | Optional | The ID of the refunded tender.<br>**Constraints**: *Maximum Length*: `192` |
 | `created_at` | `String` | Optional | The timestamp for when the refund was created, in RFC 3339 format.<br>**Constraints**: *Maximum Length*: `32` |
 | `reason` | `String` | Required | The reason for the refund being issued.<br>**Constraints**: *Maximum Length*: `192` |
 | `amount_money` | [`Money Hash`](../../doc/models/money.md) | Required | Represents an amount of money. `Money` fields can be signed or unsigned.<br>Fields that do not explicitly define whether they are signed or unsigned are<br>considered unsigned and can only hold positive amounts. For signed fields, the<br>sign of the value indicates the purpose of the money transfer. See<br>[Working with Monetary Amounts](https://developer.squareup.com/docs/build-basics/working-with-monetary-amounts)<br>for more information. |