Update JsonSerializerDefaults.Strict documentation with detailed behavior

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

xml/System.Text.Json/JsonSerializerDefaults.xml

@@ -79,7 +79,23 @@ This option implies that property names are treated as case-sensitive and that "
       </ReturnValue>
       <MemberValue>2</MemberValue>
       <Docs>
-        <summary>Specifies that stricter policies should be applied when deserializing from JSON.</summary>
+        <summary>
+          <para>Option values appropriate for strict JSON parsing.</para>
+          <para>This member implies that:</para>
+          <para>- Unmapped JSON properties are disallowed.</para>
+          <para>- Duplicate JSON properties aren't allowed.</para>
+          <para>- Nullable reference type annotations are respected.</para>
+          <para>- Required constructor parameters are respected.</para>
+        </summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+This option configures stricter deserialization behavior that rejects unmapped properties, duplicate properties, and enforces both nullable annotations and required constructor parameters.
+
+          ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="Web">