Problem documenting attributes described by an interface

[gvanbrakel]

In FF issue https://github.com/ibissource/iaf/issues/4492 some attributes are declared to be absent, although they do exist and are documented in an implemented interface

[mhdirkse]

The Frank!Doc creates placeholders for methods that are not declared explicitly but should be considered because an interface is implemented. Such a placeholder is a MultiplyInheritedMethodPlaceholder. To create these, only methods that exist in ancestor classes are considered. If a method is only inherited from an interface and has a default implementation there, then no MultiplyInheritedMethodPlaceholder is created. We should create the placeholders also for these interface-only methods.