Clarify the correct ways to differentiate different behavioral modes of API resources

We've had a number of questions during API design for service-catalog about using magic values. I believe the prevailing consensus amongst the owners of the project is that magic values in APIs are undesirable, but the API convention document does not give any treatment to this topic. Varations of how this has come up:

- Magic values for the names of objects as used in reference; ie, when a field that holds a reference to a k8s resource takes on a special value, the behavior of the enclosing resource changes
- Magic values for non-reference fields

I think it would be great to document this and get clarity for newcomers to the project, but would like to see some agreement in this issue before I file a PR.

Start by reading the API convention document and reviewing the issue's examples of magic values in references and non-reference fields. Seek agreement on the guidance before drafting; done means the document clearly explains how to represent different behavioral modes without ambiguous magic values.

Written by the indexing model from the issue text.