Undocumented behavior of REST API on schema reference

[eason-yuchen-liu]

When an Avro schema references another schema, it is undocumented on the website which GET request can retrieve the reference relationship. After a bit looking around, I think this API (GET /subjects/(string: subject)/versions/(versionId: version)) actually returns the reference relationship in the references field. However, this behavior is not documented on the website. It is also true for this API GET /schemas/ids/{int: id}.