DOCS-14158 Adding blurb to cover expression operator and upsert flag.

Author: ianf-mongodb

source/reference/method/db.collection.update.txt

@@ -310,6 +310,14 @@ If the :ref:`\<update\> <update-parameter>` document contains *only*
 
 For an example, see :ref:`example-update-multi`.
 
+Using ``$expr`` in an Update with ``Upsert``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Attempting to use the :manual:`$expr </reference/operator/query/expr/>` 
+operator with the upsert flag set to ``true`` will generate an error.
+
+.. _update-sharded-collection:
+
 Sharded Collections
 ~~~~~~~~~~~~~~~~~~~