Documentation: document the "Mixed" Data Source (#18398)

docs/sources/features/datasources/_index.md

@@ -34,6 +34,7 @@ The following datasources are officially supported:
 * [Microsoft SQL Server (MSSQL)]({{< relref "mssql.md" >}})
 * [OpenTSDB]({{< relref "opentsdb.md" >}})
 * [Testdata]({{< relref "testdata.md" >}})
+* [Mixed]({{< relref "mixed.md" >}})
 
 ## Data source plugins
 

docs/sources/features/datasources/mixed.md

++++
+title = "Using the Mixed Data Source in Grafana"
+keywords = ["grafana", "dashboard", "documentation", "panels", "mixeddata"]
+type = "docs"
+[menu.docs]
+name = "MixedData"
+parent = "datasources"
+weight = 20
++++
+
+# Mixed Data Source
+
+This Data Source allows you to specify data source on a per query basis.
+
+## Usage
+
+To use the Mixed Data Source, create a new panel or edit an existing panel and navigate to the Queries tab. From the Query popup, select "-- Mixed --". Click "Add Query" and select the desired source.
+
+You cannot change an existing query to use the Mixed Data Source.