Add explanation for "Generics return type resolution" in migration guide

[tkobayas]

Add more explanation to https://docs.drools.org/latest/drools-docs/drools/migration-guide/index.html#_generics_return_type_resolution

A) Vehicle.getMotor() is defined to return TEngine. Drools can get that TEngine is bound to Engine class, but Engine doesn't have property "adBlueRequired". Hence results in the error. "#" is syntax to specify the sub class.

B) DieselCar.getEngine() is defined to return DieselEngine. So the rule can access "adBlueRequired".

exec-model generates java codes from rules (for faster KiePackage/KieBase build), so those static class information is crucial while non-exec-model can resolve the value at rule execution time.