Parameter documentation (info) not showing in Visual Studio Code

Question

I'm just wondering how it is that I can't see the parameter documentation on String.Format (C#) in Visual Studio Code.

Here's the example of Console.WriteLine (using Ctrl+Shift+Space):

Here's the example "not working" using String.Format:

I'm fairly new to Visual Studio Code. Perhaps I missed something. If someone can clarify this then that would be much appreciated.

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EDIT: Visual Studio Code version: 1.15.1. C# for Visual Studio Code (powered by OmniSharp) version: 1.12.1.