DOCS-13063 Clarify $group order behaviour

Author: npentrel

source/includes/note-group-pipeline-sort-order.rst

@@ -43,11 +43,10 @@ For more information on expressions, see :ref:`aggregation-expressions`.
 Behavior
 --------
 
-When using :group:`$first` in a :pipeline:`$group` stage, the
-:pipeline:`$group` stage should follow a :pipeline:`$sort` stage to
-have the input documents in a defined order.
-
-.. include:: /includes/note-group-pipeline-sort-order.rst
+When using :group:`$first` in a :pipeline:`$group` stage, the output
+value depends on the order of the documents coming into pipeline. To
+guarantee a defined order, the :pipeline:`$group` pipeline stage should
+follow a :pipeline:`$sort` stage.
 
 Example
 -------

source/reference/operator/aggregation/first.txt

@@ -43,11 +43,10 @@ For more information on expressions, see :ref:`aggregation-expressions`.
 Behavior
 --------
 
-When using :group:`$last` in a :pipeline:`$group` stage, the
-:pipeline:`$group` stage should follow a :pipeline:`$sort` stage to
-have the input documents in a defined order.
-
-.. include:: /includes/note-group-pipeline-sort-order.rst
+When using :group:`$last` in a :pipeline:`$group` stage, the output
+value depends on the order of the documents coming into pipeline. To
+guarantee a defined order, the :pipeline:`$group` pipeline stage should
+follow a :pipeline:`$sort` stage.
 
 Example
 -------

source/reference/operator/aggregation/last.txt

@@ -29,6 +29,15 @@ Definition
 
    For more information on expressions, see :ref:`aggregation-expressions`.
 
+Behavior
+--------
+
+When using :group:`$push` in a :pipeline:`$group` stage, the order of
+the documents in the output array is determined by the order of the
+documents coming into pipeline. To guarantee a defined order, the
+:pipeline:`$group` pipeline stage should follow a :pipeline:`$sort`
+stage.
+
 Example
 -------
 
@@ -52,7 +61,8 @@ compute the list of items and quantities sold for each group:
 
    db.sales.aggregate(
       [
-        {
+      { $sort: { date: 1, item: 1 } },
+      {
           $group:
             {
               _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } },