DOCS-16258-setWindowFields-update (#5186) (#5214)

Co-authored-by: jason-price-mongodb <[email protected]>

Author: jason-price-mongodb

source/reference/operator/aggregation/setWindowFields.txt

@@ -228,6 +228,9 @@ The :pipeline:`$setWindowFields` stage appends new fields to existing
 documents. You can include one or more :pipeline:`$setWindowFields`
 stages in an aggregation operation.
 
+The ``$setWindowFields`` stage doesn't guarantee the order of the
+returned documents.
+
 .. _setWindowFields-window-operators:
 
 Window Operators