Update System.Drawing unsupported warning for .Net Core

[The remarks for the System.Drawing namespace say:](https://docs.microsoft.com/en-us/dotnet/api/system.drawing?view=netcore-3.0#remarks)

> Caution
> Classes within the System.Drawing namespace are not supported for use within a Windows or ASP.NET service. Attempting to use these classes from within one of these application types may produce unexpected problems, such as diminished service performance and run-time exceptions. For a supported alternative, see Windows Imaging Components.

This warning seems to be specific to .Net Framework and Windows, so it leaves the question of support of System.Drawing on .Net Core and on Unix open.

I think this warning should be updated, to make it clear what the support status of System.Drawing on .Net Core is.