Ensure all our public APIs have XML docs

[Youssef1313]

As an example:

https://github.com/microsoft/testfx/blob/b10bda33dc1682c4d6c8be903cdd9d2f52407564/src/Platform/Microsoft.Testing.Platform/OutputDevice/WarningMessageOutputDeviceData.cs

This doesn't have XML doc comment. We should fail the build when this happens. I think CS1591 is used for that but it's currently added to NoWarn. However, when I tried to remove this NoWarn locally, build still succeeds which is questionable.