(DOCSP-7079): Clarifying lookup limitation on filter page

Author: jeff-allen-mongo

source/filter-documents.txt

@@ -521,5 +521,8 @@ chart displays the the :guilabel:`Sum` of all
 Limitations
 ~~~~~~~~~~~
 
-The :pipeline:`$lookup` operator is not supported in the
-aggregation pipeline builder.
+The :pipeline:`$lookup` operator is not supported in aggregation queries
+defined in the Chart Builder. However, you can use this operator
+in Data Source pipelines. To learn more about how to use pipelines 
+to pre-process data before it reaches the Chart Builder, see
+:ref:`data-sources-pipeline`.