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

This was the topic of many discussions around OpenAPI 2.0 and earlier Swagger specifications. Here are some of them:

• Make it clear that additionalProperties can only have a schema value Make it clear that additionalProperties can only have a schema value #668
• JSON Schema Support Proposal JSON Schema Support Proposal #741
• State whether Open API schema object definitions are open or closed for extension State whether Open API schema object definitions are open or closed for extension #715
• NullPointerException in parser with additionalProperties: false (swagger-api/swagger-core#1437)

OpenAPI 2.0 assumed additionalProperties: false as the default, and disallowed explicit use of a boolean additionalProperties value. This was never fully documented, only stated in out-of-band comments. And those comments were never really reconciled with allOf semantics, which magically continued to work as though additionalProperties: true were the default.

I wasn't aware, and I'm guessing that some others also weren't aware, that this changed intentionally in the 3.0 spec. The current 3.0.1 spec explicitly states that boolean values are allowed, but doesn't explicitly state that the default is now additionalProperties: true.

I think it's worth making this explicit now, because anyone who got caught up in the 2.0 additionalProperties confusion is likely to need some assurance that this change is intentional, and not subject to informal amendments or clarifications made outside of the spec.

@handrews and I were both looking for that assurance, and couldn't find it. So I'm suggesting this small change.