Add documentation to get the rigth analysis scope

[Corniel]

To get the nice squiggly lines under issues reported by .NET project file analyzers, the Analysis scope should be set correctly in VS. We should have documentation for it: https://learn.microsoft.com/en-us/visualstudio/code-quality/configure-live-code-analysis-scope-managed-code?view=vs-2022#custom-analysis-scope