DOCS-5913 findOne if no match

Author: kay-kim

source/reference/method/db.collection.findOne.txt

@@ -14,7 +14,8 @@ Definition
    returns the first document according to the :term:`natural order`
    which reflects the order of documents on the disk. In :term:`capped
    collections <capped collection>`, natural order is the same as
-   insertion order.
+   insertion order. If no document satisfies the query, the method
+   returns null.
 
    .. include:: /includes/apiargs/method-db.collection.findOne-param.rst