Only show public interfaces in generated syntax

[dotMorten]

This addresses the issue with the generated syntax showing internal interfaces, as described in #3154
The changed method is only used to generate syntax, so should be safe. Since interfaces can only be public or internal, there's no need to check for any of the other accessibilities.

[KalleOlaviNiemitalo]

interfaces can only be public or internal

Not true

public class Outer
{
    private protected interface IPrivateProtected
    {
    }

    public class Inner : IPrivateProtected
    {
    }
}

[dotMorten]

@KalleOlaviNiemitalo From what I can tell, the effect is the same though. We still don't want to doc that interface

[KalleOlaviNiemitalo]

You'd want to doc protected interfaces though. Does this PR handle those right?

[dotMorten]

Does this PR handle those right?

It does now ;-)

[KalleOlaviNiemitalo]

How about this case:

internal class Outer
{
    public interface INested
    {
    }
}

public class Impl : Outer.INested
{
}

Here, I think you wouldn't want to mention Outer.INested in the docs, even though it is public itself.

[dotMorten]

@KalleOlaviNiemitalo Ugh I'm starting to not like you very much :-) J/K

Fixed this case now.

[yufeih]

Thanks @dotMorten for the fix and @KalleOlaviNiemitalo for the excellent code review!