Docs: Update _index.md (#25121)

docs/sources/features/datasources/_index.md

@@ -35,6 +35,14 @@ The following data sources are officially supported:
 * [Prometheus]({{< relref "prometheus.md" >}})
 * [Testdata]({{< relref "testdata.md" >}})
 
+In addition to the data sources that you have configured in your Grafana, there are three special data sources available:
+
+- **Grafana -** A built-in data source that generates random walk data. Useful for testing visualizations and running experiments.
+- **Mixed -** Select this to query multiple data sources in the same panel. When this data source is selected, Grafana allows you to select a data source for every new query that you add.
+  * The first query will use the data source that was selected before you selected **Mixed**.
+  * You cannot change an existing query to use the Mixed Data Source.
+- **Dashboard -** Select this to use a result set from another panel in the same dashboard.
+
 ## Data source plugins
 
 Since Grafana 3.0 you can install data sources as plugins. Check out [Grafana.com/plugins](https://grafana.com/plugins) for more data sources.