docs: document the OpenApiVersion property (#1189)

Author: keetonian

versions/2016-10-31.md

@@ -233,6 +233,7 @@ AccessLogSetting | [CloudFormation AccessLogSetting property](https://docs.aws.a
 CanarySetting | [CloudFormation CanarySetting property](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-apigateway-stage-canarysetting.html) | Configure a Canary Setting to a Stage of a regular deployment. This value is passed through to Cloudformation, so any value supported by `CanarySetting` is supported here.
 TracingEnabled | `boolean` | Indicates whether active tracing with X-Ray is enabled for the stage.
 Models | `List of JSON or YAML objects` | JSON schemas that describes the models to be used by API methods.
+OpenApiVersion | `string` | Version of OpenApi to use. This can either be `'2.0'` for the swagger spec or one of the OpenApi 3.0 versions, like `'3.0.1'`. Setting this property to any valid value will also remove the stage `Stage` that SAM creates.
 
 ##### Return values