Fix renderer discoverer implicit name

See mfkl/libvlcsharp-samples#10 (closed) and mfkl/libvlcsharp-samples@dce768ca and 10d1c416

Compilation directives aren't enough for shared code running in netstandard 2.0 and netstandard 2.0 has no way to distinguish between iOS and Android.

I'm not a fan of hard coding libVLC.RendererList.Last().Name but it works (for now) and the user always has the other constructor in case they want to specify the service discovery protocol themselves.

Note: mdns should not be returned by libvlc on Apple platforms, as it does not work.

Added documentation as well (more cherry picks incoming).