unity intellisense docs
Currently, the Unity experience does not provide
IntelliSense documentation for the .NET Framework.
Steps to fix:
1. find the version Unity uses by looking at its version in
C:\Program Files\Unity\Hub\Editor\VERSION\Editor\Data\NetStandard\ref\2.1.0\netstandard.dll
2. find closest NuGet package that matches the version, in this case it's
https://www.nuget.org/packages/NETStandard.Library.Ref/2.1.0
3. download it and copy netstandard.xml from it next to netstandard.dll
4. restart Visual Studio