DOCS-13966 clarify behavior of zero for $arrayElemAt idx

Author: andf-mongodb

source/reference/operator/aggregation/arrayElemAt.txt

@@ -26,19 +26,21 @@ Definition
       { $arrayElemAt: [ <array>, <idx> ] }
 
    The ``<array>`` expression can be any valid :ref:`expression
-   <aggregation-expressions>` as long as it resolves to an array.
+   <aggregation-expressions>` that resolves to an array.
 
    The ``<idx>`` expression can be any valid :ref:`expression
-   <aggregation-expressions>` as long as it resolves to an integer.
+   <aggregation-expressions>` that resolves to an integer:
 
-   - If positive, :expression:`$arrayElemAt` returns the element at the
-     ``idx`` position, counting from the start of the array.
+   - If the ``<idx>`` expression resolves to zero or a positive integer,
+     :expression:`$arrayElemAt` returns the element at the ``idx``
+     position, counting from the start of the array.
 
-   - If negative, :expression:`$arrayElemAt` returns the element at the
-     ``idx`` position, counting from the end of the array.
+   - If the ``<idx>`` expression resolves to a negative integer,
+     :expression:`$arrayElemAt` returns the element at the ``idx``
+     position, counting from the end of the array.
 
-   If the ``idx`` exceeds the array bounds, :expression:`$arrayElemAt`
-   does not return any result.
+   If ``idx`` exceeds the array bounds, :expression:`$arrayElemAt` does
+   not return a result.
 
    For more information on expressions, see
    :ref:`aggregation-expressions`.