Fix JsonIgnoreCondition.WhenWritingNull doc

[lilinus]

Doc only

It acutally applies to nullable value types too

using System;
using System.Text.Json;
using System.Text.Json.Serialization;

Forecast forecast = new()
{
    Date = null,
};
JsonSerializerOptions options = new()
{
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
};
string forecastJson = JsonSerializer.Serialize<Forecast>(forecast, options);
Console.WriteLine(forecastJson); // Prints "{}"

public class Forecast
{
    public DateTime? Date { get; set; }
};

Should I manually make PR to https://github.com/dotnet/docs ?

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[github-actions]

Holistic Review

Motivation: The XML documentation for JsonIgnoreCondition.WhenWritingNull stated the condition applies "only to reference-type properties and fields." This is inaccurate: WhenWritingNull also applies to Nullable<T> value-type members, which the PR author demonstrates with a DateTime? repro that serializes to {}.

Approach: A single-line comment change updates the summary text to "This is applied only to reference and nullable value-type properties and fields." No behavioral code is touched.

Summary: The correction accurately reflects runtime behavior. In JsonPropertyInfo, the WhenWritingNull handling is gated on PropertyTypeCanBeNull, which is true for both reference types and Nullable<T>, so null-valued nullable value-type members are indeed ignored under this condition. The wording is clear and grammatically correct. This is a low-risk, documentation-only improvement with no compatibility, performance, or test implications. LGTM.

Note: as the author mentions, equivalent conceptual documentation may also live in dotnet/docs; that is out of scope for this repository change.

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