Make Server Variable Object's properties more strict

Followup to #1809, now that we allow breaking changes.

Author: webron

versions/3.1.0.md

@@ -424,8 +424,8 @@ An object representing a Server Variable for server URL template substitution.
 
 Field Name | Type | Description
 ---|:---:|---
-<a name="serverVariableEnum"></a>enum | [`string`] | An enumeration of string values to be used if the substitution options are from a limited set. The array SHOULD NOT be empty.
-<a name="serverVariableDefault"></a>default | `string` |  **REQUIRED**. The default value to use for substitution, which SHALL be sent if an alternate value is _not_ supplied. Note this behavior is different than the [Schema Object's](#schemaObject) treatment of default values, because in those cases parameter values are optional. If the [`enum`](#serverVariableEnum) is defined, the value SHOULD exist in the enum's values.
+<a name="serverVariableEnum"></a>enum | [`string`] | An enumeration of string values to be used if the substitution options are from a limited set. The array MUST NOT be empty.
+<a name="serverVariableDefault"></a>default | `string` |  **REQUIRED**. The default value to use for substitution, which SHALL be sent if an alternate value is _not_ supplied. Note this behavior is different than the [Schema Object's](#schemaObject) treatment of default values, because in those cases parameter values are optional. If the [`enum`](#serverVariableEnum) is defined, the value MUST exist in the enum's values.
 <a name="serverVariableDescription"></a>description | `string` | An optional description for the server variable. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
 
 This object MAY be extended with [Specification Extensions](#specificationExtensions).