JsonSerializer: Document on class page whether it is thread-safe

### Type of issue

Missing information

### Description

Document on the class page whether the [JsonSerializer class](https://learn.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializer?view=net-10.0) is thread-safe.

[JSON serialization and deserialization in .NET - overview](https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/overview#thread-safety) says, "The System.Text.Json serializer was designed with thread safety in mind. [...]", but does not explicitly say JsonSerializer is thread-safe. In any event, this information should be present directly on the class page.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializer?view=net-10.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs-temp/blob/live/xml/System.Text.Json/JsonSerializer.xml

### Document Version Independent Id

99121c51-46df-981d-8777-e0cd45e4c98d

### Platform Id

1e796b95-95c4-4469-304f-c871bf18f01d

### Article author

@dotnet-bot