docs: Elaborate on index build time

Author: paveltiunov

docs/pages/product/caching/using-pre-aggregations.mdx

@@ -290,7 +290,7 @@ second.
 
 When you define additional indexes, you don't incur any additional costs on
 the data warehouse side. However, the pre-aggregation build time for a
-particular pre-aggregation increases with each index.
+particular pre-aggregation increases with each index because all indexes for pre-aggregation are built during ingestion time.
 
 ### When to use indexes?