docs: clarify usage of cron + sql in refresh_key (#9082)

* docs: clarify usage of cron + sql in refresh_key

* Update pre-aggregations.mdx

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

@@ -926,6 +926,12 @@ To have a pre-aggregation rebuild at a specific time of day, you can use a CRON
 
 You can also use `every` with `sql`:
 
+<WarningBox>
+
+Using CRON strings and `sql` is not supported and will result in a compilation error.
+
+</WarningBox>
+
 <CodeTabs>
 
 ```javascript
@@ -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