docs: Add Generic Action info on returning a list of resources (#1676)

Co-authored-by: Chaz Watkins <chaz.watkins@ericsson.com>

documentation/topics/actions/generic-actions.md

@@ -81,6 +81,17 @@ end
 >   run # ...
 > end
 > ```
+>
+> For returning many instances of the resource, you can set your action's return type to
+> `{:array, :struct}` and set the `items` constraint to the name of your resource.
+>
+> ```elixir
+  action :list_resources, {:array, :struct} do
+    constraints items: [instance_of: __MODULE__]
+
+    run # ...
+  end
+> ```
 
 ## Calling Generic Actions