(DOCSP-24291): Document aggregation query limitation for $function operator (#576)

* Add note about  operator syntax

* copy review feedback

Author: davidhou17

source/aggregation-pipeline.txt

@@ -101,8 +101,22 @@ accordingly.
 Limitations
 ~~~~~~~~~~~
 
-The :pipeline:`$lookup` operator is not supported in aggregation queries
-defined in the Chart Builder. However, you can use this operator
-in Data Source pipelines. To learn more about how to use pipelines 
-to pre-process data before it reaches the Chart Builder, see
-:ref:`data-sources-pipeline`.
+- |charts-short| doesn't support the :pipeline:`$lookup` operator in aggregation 
+  queries. 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`.
+
+- |charts-short| supports the :expression:`$function` operator only when 
+  you define the function body in a single line and 
+  wrap it in double quotes. To use this operator in your aggregation 
+  queries, you must use the following syntax:
+
+  .. code-block:: javascript
+
+     {
+       $function: {
+         body: "function(arg1, arg2, ...) { ... }",
+         args: <array expression>,
+         lang: "js"
+       }
+     }