docs: Notes on multi-stage calculations

Author: igorlukanin

docs/pages/product/data-modeling/concepts/multi-stage-calculations.mdx

@@ -15,6 +15,13 @@ engine][link-tesseract]. Tesseract is currently in preview. Use the
 
 </WarningBox>
 
+<WarningBox>
+
+Multi-stage calculations are not currently accelerated by pre-aggregations.
+Please track [this issue](https://github.com/cube-js/cube/issues/8487).
+
+</WarningBox>
+
 Common uses of multi-stage calculations:
 
 - [Period-to-date](#period-to-date) calculations, e.g., year-to-date (YTD) analysis.
@@ -332,6 +339,8 @@ data across various dimensions.
   type: rank
 ```
 
+You can reduce by one or more dimensions.
+
 ### Example
 
 Data model: