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

Author: ianf-mongodb

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

@@ -233,6 +233,12 @@ privileges.
 Behavior
 --------
 
+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