DOCS-475 nonAtomic doc for mapReduce

Author: kay-kim

source/reference/command/mapReduce.txt

@@ -129,10 +129,11 @@ mapReduce
 
           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.
+          <db.collection.update()>`. If ``true``, post processing step
+          will not be executed while locking the database, and partial
+          results will be visible as they are processed. The option is
+          valid only for ``merge`` and ``reduce`` output operations for
+          which post processing can be a long operation.
 
    .. seealso:: ":method:`mapReduce()`" and ":term:`map-reduce`."
 

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

@@ -159,10 +159,11 @@ db.collection.mapReduce()
 
           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.
+          <db.collection.update()>`. If ``true``, post processing step
+          will not be executed while locking the database, and partial
+          results will be visible as they are processed. The option is
+          valid only for ``merge`` and ``reduce`` output operations for
+          which post processing can be a long operation.
 
    .. seealso:: :term:`map-reduce`, provides a greater overview
       of MongoDB's map-reduce functionality.