DOCS-475 nonAtomic doc

Author: kay-kim

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

@@ -153,6 +153,17 @@ db.collection.mapReduce()
                    shard the results collection according to the
                    ``_id`` field.
 
+   :param nonAtomic: 
+
+          .. versionadded:: 2.1
+
+          Optional. Specify output operation as non-atomic such that
+          the output behaves like a normal ``multi`` :method:`update()
+          <db.collection.update()>`. The option is valid only for
+          ``merge`` and ``reduce`` output operations. If ``true``, post
+          processing step will not be executed under the write lock,
+          and partial results will be visible as they are processed.
+
    .. seealso:: :term:`map-reduce`, provides a greater overview
       of MongoDB's map-reduce functionality.