mdoc no longer documents private interfaces

Resolves this reported bug: 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).
Good point on the `else if`, I'll make that change.

As for the `.GetMethod` ... I didn't consider that there might be a set only property. I will add a test case and adjust the code to make sure it works in both cases.