https://bugzilla.xamarin.com/show_bug.cgi?id=18411
If a class explicitly implements an internal interface, those members will no
longer be documented, nor will the interface show up in the class' signature.
This allows for a library to hide internal implementation details that an end
user would not be able to take advantage of (ie. can't cast to internal
interface to call methods).
Includes test (check-monodocer-internal-interface) added to makefile.