Clarify how setExtension and getExtension work with Schema properties

[Azquelt]

Schema extends Extensible which suggests that extension properties should start with x-.

However, schemas actually permit properties with any name and has set and get methods to allow this.

We should clarify which arbitrary properties of a schema are considered extensions.

[Azquelt]

If we decide to allow getExtension and setExtension to manage any unknown properties, we should maybe redeclare those methods on Schema and/or extend the wording in Extensible to indicate that some model objects may have different rules for extension names.