DOCSP-24216 aggregation update (#1584)

Author: jason-price-mongodb

source/includes/aggregation-pipeline-introduction.rst

@@ -9,4 +9,15 @@ An aggregation pipeline consists of one or more :ref:`stages
   stage.
 
 - An aggregation pipeline can return results for groups of documents.
-  For example, return the total, average, maximum, and minimum values.
+  For example, return the total, average, maximum, and minimum values.
+
+Starting in MongoDB 4.2, you can update documents with an aggregation
+pipeline if you use the stages shown in
+:doc:`/tutorial/update-documents-with-aggregation-pipeline`.
+
+.. note::
+
+   Aggregation pipelines run with the
+   :method:`db.collection.aggregate()` method do not modify documents in
+   a collection, unless the pipeline contains a :pipeline:`$merge` or
+   :pipeline:`$out` stage.