Ensure our cross-platform docs are not Framework-centric and Windows-centric any longer

We migrated a lot of our legacy content which now applies to more than .NET Framework. This issue applies to both conceptual content and API reference.

We need to review the docs to make sure they don't mention Framework/Windows only, when they apply to other products and platforms and that samples work cross-platform. This might become a larger project.

For example:
- https://docs.microsoft.com/en-us/dotnet/api/system.io.textwriter.write?view=netframework-4.7#System_IO_TextWriter_Write_System_String_System_Object_System_Object_System_Object_