State explicitly: additionalProperties : true is the default

Address OAI#715 - State (explicitly) whether Open API schema object definitions are open or closed for extension.

Author: tedepstein

versions/3.0.2.md

@@ -2301,7 +2301,7 @@ The following properties are taken from the JSON Schema definition but their def
 - not - Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema.
 - items - Value MUST be an object and not an array. Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema. `items` MUST be present if the `type` is `array`.
 - properties - Property definitions MUST be a [Schema Object](#schemaObject) and not a standard JSON Schema (inline or referenced).
-- additionalProperties - Value can be boolean or object. Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema.
+- additionalProperties - Value can be boolean or object. Inline or referenced schema MUST be of a [Schema Object](#schemaObject) and not a standard JSON Schema. Consistent with JSON Schema, `additionalProperties` defaults to `true` or empty schema, allowing additional properties with any legal value.
 - description - [CommonMark syntax](http://spec.commonmark.org/) MAY be used for rich text representation.
 - format - See [Data Type Formats](#dataTypeFormat) for further details. While relying on JSON Schema's defined formats, the OAS offers a few additional predefined formats.
 - default - The default value represents what would be assumed by the consumer of the input as the value of the schema if one is not provided. Unlike JSON Schema, the value MUST conform to the defined type for the Schema Object defined at the same level. For example, if `type` is `string`, then `default` can be `"foo"` but cannot be `1`.