Document JsonSerializerDefaults.Strict behavior

[Copilot]

The JsonSerializerDefaults.Strict documentation was vague ("stricter policies should be applied when deserializing from JSON").

Changes

Updated xml/System.Text.Json/JsonSerializerDefaults.xml to document the four specific behaviors configured by Strict:

• Unmapped JSON properties are disallowed (UnmappedMemberHandling.Disallow)
• Duplicate JSON properties aren't allowed (AllowDuplicateProperties = false)
• Nullable reference type annotations are respected (RespectNullableAnnotations = true)
• Required constructor parameters are respected (RespectRequiredConstructorParameters = true)

Documentation format now matches JsonSerializerDefaults.Web with bulleted summary and remarks section.

Original prompt

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This section details on the original issue you should resolve

<issue_title>JsonSerializerDefaults.Strict does not document its behavior</issue_title>
<issue_description>The current documentation for JsonSerializerDefaults.Strict contains the rather vague

Specifies that stricter policies should be applied when deserializing from JSON.

We should expand this to include details, similar to how we already do this for JsonSerializerDefaults.Web. For reference here is the actual implementation:

https://github.com/dotnet/runtime/blob/0de4dab981f8bf335eeee4b8c81f44893be019a5/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonSerializerOptions.cs#L182-L185</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes JsonSerializerDefaults.Strict does not document its behavior #12254

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[Copilot]

Pull request overview

This PR clarifies the documentation for JsonSerializerDefaults.Strict by replacing a vague description with a concrete list of the behaviors it enables, aligning the format with JsonSerializerDefaults.Web.

Changes:

• Expanded Strict member docs to list the specific strictness behaviors it implies.
• Added a markdown-based remarks section describing the strict deserialization behavior.
• Adjusted the summary structure to match the existing Web member documentation style.

[Copilot]

The bullet list mixes phrasing styles ("are disallowed" vs "aren't allowed"). Consider using consistent wording across all bullets (for example, use "are disallowed" for both).

Suggested change

	<para>- Duplicate JSON properties aren't allowed.</para>
	<para>- Duplicate JSON properties are disallowed.</para>

[gewarren]

Preview looks good.