small change to resource page

docs/website/docs/general-usage/resource.md

@@ -8,7 +8,7 @@ keywords: [resource, api endpoint, dlt.resource]
 
 ## Declare a resource
 
-A [resource](glossary.md#resource) is a function that yields data. To create a
+A [resource](glossary.md#resource) is a ([optionally async](https://dlthub.com/docs/reference/performance#parallelism)) function that yields data. To create a
 resource, we add the `@dlt.resource` decorator to that function.
 
 Commonly used arguments: