DOCS-14310: Add output order disclaimer. (#495) (#561)

* DOCS-14310: Add output order disclaimer.

* DOCS-14310: Update to include PR review edits

Co-authored-by: Sarah Olson <sarah.olson@Sarahs-MacBook-Pro.local>

Co-authored-by: Sarah Olson <sarah.olson@Sarahs-MacBook-Pro.local>

Author: sarah-olson-mongodb

source/reference/operator/aggregation/unionWith.txt

@@ -17,10 +17,14 @@ Definition
 
    .. versionadded:: 4.4
 
-   Performs a union of two collections; i.e.
-   :pipeline:`$unionWith` combines pipeline results from two
-   collections into a single result set. The stage outputs the combined
-   result set (including duplicates) to the next stage.
+   Performs a union of two collections. :pipeline:`$unionWith` combines 
+   pipeline results from two collections into a single result set. The stage 
+   outputs the combined result set (including duplicates) to the next stage.
+
+   The order in which the combined result set documents are output is 
+   unspecified.
+
+
 
 Syntax
 ------