docs: Clarify data source detection in the SQL Runner

docs/pages/product/workspace/sql-runner.mdx

@@ -35,13 +35,22 @@ and click <Btn>▶ Run</Btn>. The query results will be displayed under
   src="https://ucarecdn.com/2f910abf-5b94-43cd-87f8-14c86bd98f77/"
 />
 
-## Querying a data source vs Cube Store
+## Querying data sources
 
-The SQL Runner can run queries against any configured data sources, which is
-helpful for diagnosing database-specific issues. It can also run queries against
-Cube Store, which is useful for testing pre-aggregations directly to see if they
-return expected results. You can switch data source(s) by clicking the dropdown
-under **Data Source**:
+SQL Runner can run queries against configured data sources, which is
+helpful for diagnosing database-specific issues.
+
+<InfoBox>
+
+In order for an additional data source to show up in SQL Runner, it needs to be
+the [data source][ref-cube-datasource] in at least one cube definition.
+
+</InfoBox>
+
+It can also run queries against Cube Store, which is useful for testing
+pre-aggregations directly to see if they return expected results.
+
+You can switch data source(s) by clicking the dropdown under <Btn>Data Source</Btn>:
 
 <Screenshot
   highlight="inset(25% 60% 53% 6% round 10px)"
@@ -97,6 +106,8 @@ in a data model should be a `numeric`).
   src="https://ucarecdn.com/c2735f7c-d5e7-4b6b-903b-8d0db07b668e/"
 />
 
+
 [cubecloud-signup]: https://cubecloud.dev/auth/signup
 [ref-conf-scheduled-refresh-ctx]:
   /reference/configuration/config#scheduledrefreshcontexts
+[ref-cube-datasource]: /reference/data-model/cube#data_source