Add note about allowing out of order metadata properties (#10587)

Author: gewarren

xml/System.Text.Json.Serialization/ReferenceHandler.xml

@@ -135,11 +135,11 @@
  
 * For the metadata properties within the JSON to be considered well-formed, they must follow these rules:
 
-  * The `$id` metadata property must be the first property in the JSON object.
+  * Unless <xref:System.Text.Json.JsonSerializerOptions.AllowOutOfOrderMetadataProperties> is set to `true`, the `$id` metadata property must be the first property in the JSON object.
   * A JSON object that contains a `$ref` metadata property must not contain any other properties.
   * The value of the `$ref` metadata property must refer to an `$id` that has appeared earlier in the JSON.
   * The value of the `$id` and `$ref` metadata properties must be a JSON string.
-  * For enumerable types, such as <xref:System.Collections.Generic.List%601>, the JSON array must be nested within a JSON object containing an `$id` and `$values` metadata property, in that order.
+  * For enumerable types, such as <xref:System.Collections.Generic.List%601>, the JSON array must be nested within a JSON object containing an `$id` and `$values` metadata property, in that order. (However, the order is not important if <xref:System.Text.Json.JsonSerializerOptions.AllowOutOfOrderMetadataProperties> is set to `true`.)
   * For enumerable types, the `$values` metadata property must be a JSON array.
   * The `$values` metadata property is only valid when referring to enumerable types.