Document new System.Text.Json features

[gewarren]

• [ ] Polymorphism - covered by https://github.com/dotnet/docs/issues/30879.
• [ ] Utf8Reader.CopyString
• [ ] Json contract customization - covered by https://github.com/dotnet/docs/issues/30878.
• [x] Json required properties (https://github.com/dotnet/runtime/issues/29861)