Add note about thread safety

docs/standard/serialization/system-text-json-overview.md

@@ -3,6 +3,7 @@ title: "Serialize and deserialize JSON using C# - .NET"
 description: This overview describes the System.Text.Json namespace functionality for serializing to and deserializing from JSON in .NET.
 ms.date: "01/10/2020"
 no-loc: [System.Text.Json, Newtonsoft.Json]
+zone_pivot_groups: dotnet-version
 helpviewer_keywords: 
   - "JSON serialization"
   - "serializing objects"
@@ -33,6 +34,27 @@ There are some limitations on what parts of the library that you can use from Vi
 
 For information about security threats that were considered when designing <xref:System.Text.Json.JsonSerializer>, and how they can be mitigated, see [`System.Text.Json` Threat Model](https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.Json/docs/ThreatModel.md).
 
+## Thread safety
+
+* The `System.Text.Json` types are thread-safe, including:
+
+:::zone pivot="dotnet-5-0,dotnet-core-3-1"
+
+* <xref:System.Text.Json.JsonSerializer>
+* <xref:System.Text.Json.Utf8JsonReader>
+* <xref:System.Text.Json.Utf8JsonWriter>
+* <xref:System.Text.Json.JsonDocument>
+:::zone-end
+
+:::zone pivot="dotnet-6-0"
+
+* <xref:System.Text.Json.JsonSerializer>
+* <xref:System.Text.Json.Utf8JsonReader>
+* <xref:System.Text.Json.Utf8JsonWriter>
+* <xref:System.Text.Json.JsonDocument>
+* `JsonNode`
+:::zone-end
+
 ## Additional resources
 
 * [How to use the library](system-text-json-how-to.md)

docs/standard/serialization/system-text-json-use-dom-utf8jsonreader-utf8jsonwriter.md

@@ -43,7 +43,9 @@ The preceding code:
 * Assumes the JSON to analyze is in a string named `jsonString`.
 * Calculates an average grade for objects in a `Students` array that have a `Grade` property.
 * Assigns a default grade of 70 for students who don't have a grade.
-* Counts students by incrementing a `count` variable with each iteration. An alternative is to call <xref:System.Text.Json.JsonElement.GetArrayLength%2A>, as shown in the following example:
+* Creates the `JsonDocument` instance in a [`using` statement](../../csharp/language-reference/keywords/using-statement.md) because `JsonDocument` implements `IDisposable`. After a `JsonDocument` instance is disposed, you lose access to all of its `JsonElement` instances also. To retain access to a `JsonElement` instance, make a copy of it before the parent `JsonDocument` instance is disposed. To make a copy, call <xref:System.Text.Json.JsonElement.Clone%2A?displayProperty=nameWithType>. For more information, see [JsonDocument is IDisposable](system-text-json-migrate-from-newtonsoft-how-to.md#jsondocument-is-idisposable).
+
+The preceding example code counts students by incrementing a `count` variable with each iteration. An alternative is to call <xref:System.Text.Json.JsonElement.GetArrayLength%2A>, as shown in the following example:
 
   :::code language="csharp" source="snippets/system-text-json-how-to/csharp/JsonDocumentDataAccess.cs" id="AverageGrades2":::
   :::code language="vb" source="snippets/system-text-json-how-to/vb/JsonDocumentDataAccess.vb" id="AverageGrades2":::