Nested `private protected` types incorrectly included in documentation about shfb

[EWSoftware]

It looks like that's a C# 7.2 feature so it wasn't around when MRefBuilder was developed. I can add support for it. However, since they are visible within the same assembly to derived classes or those granted access with InternalsVisibleTo, I suppose a case could be made for documenting them so perhaps it can be configurable but default to not showing them.

[Zastai]

I would agree that they should be included if I had configured the docs as including internal, but I have not - and ProtectedInternalAsProtected most definitely should not apply to private protected.

More importantly, it's currently inconsistent, because the issue affects only nested types, not fields/properties/constructors/methods.

But yes, a separate visibility checkbox would make sense, even if they could initially be lumped in with internal items.