Are Overlays restricted to operate on OpenAPI documents?

[ralfhandl]

Do we want to restrict Overlays to only operate on OpenAPI documents? For the most part the spec could easily be applied to any JSON or YAML doc. And in several places we seem to allow base documents of other types, e.g. target is described as

URL to the target document (such as an [[OpenAPI]] document) ...

On the other hand the original scope described in README.md is

The Overlay specification defines a way of creating documents that contain information to be merged with an OpenAPI description at some later point in time, for the purpose of updating the OpenAPI description with additional information.