DOCS-15241 constant folding and commutativity (#2532)

* DOCS-15241 constant folding and commutativity

* DOCS-15241 constant folding and commutativity

* DOCS-15241 constant folding and commutativity

* DOCS-15241 constant folding and commutativity

* DOCS-15241 constant folding and commutativity

* DOCS-15241 constant folding and commutativity

* Review feedback

Author: Dave Cuthbert

source/reference/operator/aggregation/add.txt

@@ -30,6 +30,14 @@ Definition
    either all numbers or to numbers and a date. For more information on
    expressions, see :ref:`aggregation-expressions`.
 
+   Starting in MongoDB 6.1 you can optimize the ``$add`` operation. To
+   improve performance, group references at the end of the argument
+   list. For example,
+
+   .. code-block:: javascript
+
+      $add: [ 1, 2, 3, '$a', '$b', '$c' ]
+
 Examples
 --------
 

source/reference/operator/aggregation/multiply.txt

@@ -28,6 +28,14 @@ Definition
    <aggregation-expressions>` as long as they resolve to numbers. For
    more information on expressions, see :ref:`aggregation-expressions`.
 
+   Starting in MongoDB 6.1 you can optimize the ``$multiply`` operation.
+   To improve performance, group references at the end of the argument
+   list. For example,
+
+   .. code-block:: javascript
+
+      $multiply: [ 1, 2, 3, '$a', '$b', '$c' ]
+
 Example
 -------