docs: clarify usage of cron + sql in refresh_key

Author: AvilaJulio

docs/pages/reference/data-model/pre-aggregations.mdx

@@ -924,6 +924,12 @@ cubes:
 To have a pre-aggregation rebuild at a specific time of day, you can use a CRON string with some limitations. For more details about values that can be used with the `every` parameter, please refer to the
 [`refreshKey`][ref-cube-refreshkey] documentation.
 
+<WarningBox>
+
+Using CRON strings and `sql` is not supported and will result in a compilation error.
+
+</WarningBox>
+
 You can also use `every` with `sql`:
 
 <CodeTabs>
@@ -1756,4 +1762,4 @@ cubes:
 [ref-aggregating-indexes]: /product/caching/using-pre-aggregations#aggregating-indexes
 [ref-pre-aggs]: /product/caching/using-pre-aggregations
 [ref-ref-cubes]: /reference/data-model/cube
-[ref-custom-granularity]: /reference/data-model/dimensions#granularities
+[ref-custom-granularity]: /reference/data-model/dimensions#granularities