DOCS-1114: clarify assurances of $addToSet

Author: Sam Kleinman

source/reference/operator/addToSet.txt

@@ -9,8 +9,7 @@ $addToSet
    The :operator:`$addToSet` operator adds a value to an array only *if* the
    value is *not* in the array already. If the value *is* in the
    array, :operator:`$addToSet` returns without modifying the
-   array. Otherwise, :operator:`$addToSet` behaves the same as
-   :operator:`$push`. Consider the following example:
+   array. Consider the following example:
 
    .. code-block:: javascript
 
@@ -20,5 +19,11 @@ $addToSet
    ``field``, *only if* ``value1`` is not already a member of this
    array.
 
-   .. see:: :operator:`$each`
+   .. note:: 
+      
+      :operator:`$addToSet` only ensures that there are no duplicate
+      items in a set and makes no guarantees regarding the order of
+      the elements in the set. 
+
+   .. see:: :operator:`$each` and :operator:`$push`