[OAS 3.1.0] Summary and description now valid when $ref is set

[Azquelt]

Previously, when $ref was used, nothing else should be set. Now, Reference objects allow summary and description to be set along with $ref.

We should:

• [x] Update existing documentation for $ref #618
• [x] Add tests for setting summary and description along with $ref for objects which support them #618
  • [x] Model tests
  • [x] Annotation tests, where the annotation has ref along with description and/or summary