Generate documentation for fields in the superclass of an example object

Also try to handle wildcard types like List<? extends MyClass> by treating
this as List<MyClass> when creating example object classes. And make cases
where creating example object classes fails generate a null instead of a
String.

!BUG:25422
!TESTS:
GET /groups/{id} should now document the name field as a string rather than null
and the members field as a list of objects rather than a null or a list of nulls.