[OAS][DOCS] Clarify license, version, and source URL (#2859)

Author: lcawl

docs/overlays/elasticsearch-serverless-openapi-overlays.yaml

@@ -11,7 +11,12 @@ actions:
         **Technical preview**  
         This functionality is in technical preview and may be changed or removed in a future release.
         Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.
-      license:
+
+        ## Documentation source and versions
+        
+        This documentation is derived from the `main` branch of the [elasticsearch-specification](https://github.com/elastic/elasticsearch-specification) repository.
+        It is provided under license [Attribution-NonCommercial-NoDerivatives 4.0 International](https://creativecommons.org/licenses/by-nc-nd/4.0/).
+      x-doc-license:
         name: Attribution-NonCommercial-NoDerivatives 4.0 International
         url: 'https://creativecommons.org/licenses/by-nc-nd/4.0/'
       x-feedbackLink: