- [feature] New standalone function with_polymorphic()

- [feature] New standalone function with_polymorphic()
provides the functionality of query.with_polymorphic()
in a standalone form.   It can be applied to any
entity within a query, including as the target
of a join in place of the "of_type()" modifier.
[ticket:2333]
- redo a large portion of the inheritance docs in terms
of declarative, new with_polymorphic() function
- upgrade examples/inheritance/polymorph, rename to "joined"