DOCS-475 nonAtomic option documentation

Author: kay-kim

source/reference/command/mapReduce.txt

@@ -123,6 +123,17 @@ mapReduce
                    enabled, the map-reduce operation will 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:: ":method:`mapReduce()`" and ":term:`map-reduce`."
 
       Also, the ":wiki:`MapReduce` page, provides a greater overview