Create or update conceptual docs for new System.Text.Json features in .NET 11

### Describe the issue or suggestion

Track conceptual, compatibility, and What's New documentation for user-visible `System.Text.Json` changes in .NET 11.

The source corpus is [merged `dotnet/runtime` pull requests labeled `area-System.Text.Json`](https://github.com/dotnet/runtime/pulls?q=is%3Apr+is%3Amerged+base%3Amain+label%3A%22area-System.Text.Json%22+merged%3A2025-08-10..2026-08-18) from August 10, 2025, through August 18, 2026.

## Tier 1: Major documentation features

- [ ] **1. C# union serialization.** Document union contracts, `JsonTypeInfoKind.Union`, `JsonUnionAttribute`, type-classifier APIs, reflection and source-generation support, and the final behavior for nullable and default-valued unions. Include `JsonUnionTypeStructuralClassifier`, which distinguishes cases by JSON token kind and root-level object property names. Implementation: [dotnet/runtime#128162](https://github.com/dotnet/runtime/pull/128162), [dotnet/runtime#128900](https://github.com/dotnet/runtime/pull/128900), [dotnet/runtime#131797](https://github.com/dotnet/runtime/pull/131797), [dotnet/runtime#131879](https://github.com/dotnet/runtime/pull/131879), and [dotnet/runtime#132411](https://github.com/dotnet/runtime/pull/132411).
- [ ] **2. JSON Lines serialization.** Document the four `SerializeAsyncEnumerable` overloads for `Stream` and `PipeWriter`. Explain that `topLevelValues: true` writes canonical JSONL with `\n`, while the default output remains a JSON array. Implementation: [dotnet/runtime#127567](https://github.com/dotnet/runtime/pull/127567).
- [ ] **3. F# discriminated unions.** Document the string representation for fieldless cases and the `$type` object representation for cases with fields. State prominently that support is reflection-only and doesn't support source generation or Native AOT. Implementation: [dotnet/runtime#125610](https://github.com/dotnet/runtime/pull/125610).
- [ ] **4. Closed-hierarchy polymorphism inference.** Document global opt-in through `JsonSerializerOptions.InferClosedTypePolymorphism`, source-generation configuration, and per-hierarchy configuration through `JsonPolymorphicAttribute.InferClosedTypePolymorphism`. Include precedence for explicit `[JsonDerivedType]` registrations and per-type opt-out. Implementation: [dotnet/runtime#130808](https://github.com/dotnet/runtime/pull/130808) and [dotnet/runtime#131623](https://github.com/dotnet/runtime/pull/131623).
- [ ] **5. Source generation for inaccessible members.** Update the source-generation support matrix and related guidance to cover private, internal, and protected `[JsonInclude]` members, inaccessible `[JsonConstructor]` constructors, and preservation of initializer defaults for omitted `init` properties. Remove the existing statement that private `[JsonInclude]` members aren't supported. Implementation: [dotnet/runtime#124650](https://github.com/dotnet/runtime/pull/124650), [dotnet/runtime#126507](https://github.com/dotnet/runtime/pull/126507), and [dotnet/runtime#130163](https://github.com/dotnet/runtime/pull/130163).
- [ ] **6. Naming-policy controls.** Document `JsonNamingPolicy.PascalCase`, `JsonKnownNamingPolicy.PascalCase`, and `JsonNamingPolicyAttribute` on types and members. Include the precedence order: `JsonPropertyName` → member naming-policy attribute → type naming-policy attribute → global policy → original name. Implementation: [dotnet/runtime#124645](https://github.com/dotnet/runtime/pull/124645) and [dotnet/runtime#124644](https://github.com/dotnet/runtime/pull/124644).
- [ ] **7. Type-level ignore conditions.** Document `[JsonIgnore(Condition = ...)]` on classes, structs, and interfaces, including member-level precedence, precedence over `DefaultIgnoreCondition`, and the invalid `Always` configuration. Implementation: [dotnet/runtime#124646](https://github.com/dotnet/runtime/pull/124646).
- [ ] **8. Open generic derived types in polymorphism.** Add examples for `[JsonDerivedType(typeof(Derived<>))]` on generic bases. Document supported unification patterns and the failures reported at run time or through `SYSLIB1229`. Implementation: [dotnet/runtime#127318](https://github.com/dotnet/runtime/pull/127318).
- [ ] **9. Built-in converters for new numeric types.** Add `BFloat16`, `Decimal32`, `Decimal64`, and `Decimal128` to supported-type, source-generation, and JSON Schema guidance. Include the corresponding `JsonMetadataServices` converter properties. Implementation: [dotnet/runtime#131523](https://github.com/dotnet/runtime/pull/131523).

## Tier 2: Existing-topic and compatibility updates

- [ ] **10. Open generic converters on generic types.** Include the new `[JsonConverter(typeof(OptionConverter<>))]` pattern in What's New and retain the existing conceptual guidance for direct open-generic converter use without a `JsonConverterFactory`. Implementation: [dotnet/runtime#123209](https://github.com/dotnet/runtime/pull/123209).
- [ ] **11. By-reference constructor parameters.** Document deserialization support for constructors with `in`, `ref`, `out`, and `ref readonly` parameters in reflection and source-generation modes. Explain that `out` parameters don't bind JSON constructor arguments. Implementation: [dotnet/runtime#122950](https://github.com/dotnet/runtime/pull/122950).
- [ ] **12. Extension-data improvements.** Update `handle-overflow.md` to accept `IReadOnlyDictionary` and `IReadOnlyDictionary`, including how deserialization materializes and preserves entries. Also account for corrected `JsonObject` extension-data serialization. Implementation: [dotnet/runtime#120636](https://github.com/dotnet/runtime/pull/120636) and [dotnet/runtime#122838](https://github.com/dotnet/runtime/pull/122838).
- [ ] **13. `IReadOnlySet` support.** Add `IReadOnlySet` to `supported-types.md` and describe serialization as a JSON array and deserialization to a set implementation. Include `JsonMetadataServices.CreateIReadOnlySetInfo`. Implementation: [dotnet/runtime#120306](https://github.com/dotnet/runtime/pull/120306).
- [ ] **14. Generic metadata lookup.** Document `JsonSerializerOptions.GetTypeInfo()` and `TryGetTypeInfo()`, which remove manual casts and provide a nonthrowing lookup. Implementation: [dotnet/runtime#123940](https://github.com/dotnet/runtime/pull/123940).
- [ ] **15. Reset writer output and options together.** Document `Utf8JsonWriter.Reset(Stream, JsonWriterOptions)` and `Reset(IBufferWriter, JsonWriterOptions)` with a pooling or reuse example. Implementation: [dotnet/runtime#126578](https://github.com/dotnet/runtime/pull/126578).

## Existing coverage and immediate gaps

The current .NET 11 libraries draft already covers or partly covers union serialization, JSON Lines, F# discriminated unions, closed-hierarchy inference, naming and ignore controls, `IReadOnlySet`, generic metadata lookup, and writer reset overloads. `converters-how-to.md` already covers open generic converters.

The most important corrections and omissions are:

- `source-generation-modes.md` and `immutability.md` still state that source generation can't use private `[JsonInclude]` members.
- The F# discriminated-union section doesn't state its reflection-only, trimming, and Native AOT limitations.
- The naming-policy section describes `JsonNamingPolicyAttribute` as member-level even though it also supports types.
- The closed-hierarchy section doesn't yet cover the source-generation option, per-type attribute, opt-out behavior, or associated diagnostics.
- `supported-types.md` lacks `IReadOnlySet`.
- `handle-overflow.md` lists only mutable dictionary extension-data types.
- `extract-schema.md` doesn't cover the three exporter changes.
- New numeric converters and the exception-propagation compatibility change aren't covered.

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[Associated WorkItem - 626184](https://dev.azure.com/msft-skilling/Content/_workitems/edit/626184)