Why does Visual Studio not show tooltip (Intellisense?) documentation for standard Mono classes?

I have noticed I get normal documentation while writing code for Unity specific classes such as GameObject, but not for standard Mono classes, for example List from the System namespace:

When I’m coding on a normal .NET C# project, the documentation works as expected:

I have checked and the assembly for the System namespace for Unity is from: C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v3.5\Profile\Unity Subset v3.5\System.dll but there doesn’t seem to be a System.XML file with associated documentation. Is this normal? How do I get to fix this?

It has been brought to my attention that Unity doesn’t include the System.xml file required for Intellisense documentation for their old mono runtime.

It seems from Unity 2017 onwards, as long as you enable .NET 4.6 on the API Compatibility Level setting on the Player Settings, you will get full documentation. I have tested and it worked fine.

EDIT: It seems at version 2017.2.7f and perhaps earlier it does not work again. I get Intellisense for Unity specific APIs but not for the Mono APIs.

This is happening to me in Unity 2020.3.21f1. Did you ever find a solution? I get intellisense method summaries for all unity-related methods, but only parameter info for methods under the system namespace.

Any fix on this ?? I have got the same problem… Also no tooltips in version 2020.3