Querying an association that consists of an inheritance hierarchy fails due to Hibernate Proxies

[jamesdh]

This easily happens when the inheritance hierarchy in question is an association on a parent domain model that is being queried. If the object is not already in Hibernates cache, it generates a proxy for it when being retrieved, but then GraphQL is unable to adequately determine its type when fulfilling the query.

Adding this mostly as a placeholder/reminder to submit a PR that clarifies this gotcha in the docs.