[Rollup] Add a comment about api use by the data view plugin (elastic…

…#163432)

## Summary
Adds a comment that the API endpoint /rollup/indices is used by the data
view plugin.

With elastic#162674 merged, the issue
elastic#152708 has been closed. The
Data Discovery team is not planning any further work around rollup api
use due to the feature being deprecated. I think adding a comment to the
API code directly will help us remember this dependency for when we are
going to edit or remove the endpoint.

x-pack/plugins/rollup/server/routes/api/indices/register_get_route.ts

@@ -18,6 +18,7 @@ export const registerGetRoute = ({
 }: RouteDependencies) => {
   router.get(
     {
+      // this endpoint is used by the data views plugin, see https://github.com/elastic/kibana/issues/152708
       path: addBasePath('/indices'),
       validate: false,
     },