DOCSP-39616-push-aggregation-missing-behavior (#7857)

* DOCSP-39616-push-aggregation-missing-behavior

* Editorial per feedback.

Author: mmaville-mdb

source/reference/operator/aggregation/push.txt

@@ -33,6 +33,15 @@ Syntax
 For more information on expressions, see
 :ref:`aggregation-expressions`.
 
+Behavior
+--------
+
+When using :group:`$push` in a :pipeline:`$group` stage, the order of 
+the documents coming into the pipeline determines the order of the 
+documents in the output array. To guarantee a defined order, the
+:pipeline:`$group` pipeline stage should follow a :pipeline:`$sort`
+stage.
+
 Examples
 --------