DOCSP-13735 add empty array result for setIntersection

Author: andf-mongodb

source/reference/operator/aggregation/setIntersection.txt

@@ -38,6 +38,9 @@ Behavior
 
 .. include:: /includes/important-set-operator-semantics.rst
 
+If no intersections are found (i.e. the input arrays contain no common
+elements), :expression:`$setIntersection` returns an empty array.
+
 .. include:: /includes/extracts/fact-agg-top-level-expressions-setIntersection.rst
 
 .. list-table::