Expander: expanded document is not self-contained with some circular $ref patterns

[fredbi]

Let's make things official about this limitation:

• whenever $ref form a cycle remotely (i.e. the cycle is out of the root document)

Then the expanded result leaves some $ref items to the remote document with the cycle.
This means the expanded result is not always self-contained.

At this moment, this is not blocking, and libraries such as go-openapi/validate or go-openapi/analysis still work with this.