Incorrect documentation for JsonIgnoreCondition.WhenWritingNull

The documentation for [JsonIgnoreCondition.WhenWritingNull](https://docs.microsoft.com/dotnet/api/system.text.json.serialization.jsonignorecondition?view=net-6.0#system-text-json-serialization-jsonignorecondition-whenwritingnull) reads:

> If the value is null, the property is ignored during serialization. This is applied only to reference-type properties and fields.

whereas in fact it also applies to System.Nullable, which is presumably the desired behavior.