DOCS-3286: clarification about map reduce lock during reduce phase

Author: Sam Kleinman

source/core/map-reduce-concurrency.txt

@@ -20,7 +20,9 @@ During the operation, map-reduce takes the following locks:
   collection takes a write lock.
 
 - If the output collection exists, then the output actions (i.e.
-  ``merge``, ``replace``, ``reduce``) take a write lock.
+  ``merge``, ``replace``, ``reduce``) take a write lock. This write
+  lock is *global*, and blocks all operations on the :program:`mongod`
+  instance.
 
 .. versionchanged:: 2.4
    The V8 JavaScript engine, which became the default in 2.4, allows
@@ -29,9 +31,10 @@ During the operation, map-reduce takes the following locks:
    functions) executed in a single thread.
 
 .. note::
-
    The final write lock during post-processing makes the results appear
    atomically. However, output actions ``merge`` and ``reduce`` may
    take minutes to process. For the ``merge`` and ``reduce``, the
-   ``nonAtomic`` flag is available. See the
-   :method:`db.collection.mapReduce()` reference for more information.
+   ``nonAtomic`` flag is available, which releases the lock between
+   writing each output
+   document. the :method:`db.collection.mapReduce()` reference for more
+   information.