Improve info on use of @Controller's with aop proxying

Before this change, issues surrounding the use of @Controller's in
combination with AOP proxying, resulted in an IllegalArgumentException
when trying to invoke the controller method.

This change detects such cases proactively and reports them with a
clear recommendation to use class-based proxying when it comes to
@Controller's. This is the most optimcal approach for controllers
in many respects, also allows @MVC annotations to remain on the
class.

The documentation has also been updated to have a specific section
on @Controller's and AOP proxying providing the same advice.

Issue:SPR-11281