Guidance on response modeling, including no content

[handrews]

See:

• Do we need to clarify what a response missing content means OpenAPI-Specification#3536
• v3.1.2: Clarify how to describe no response content OpenAPI-Specification#4611

There are some non-obvious things about how to best model responses, including the fact that things that aren't modeled are not necessarily forbidden. Among other things, this makes expressing "if there is content, it is an error" is rather tricky.