Update source/release-notes/2.4.txt

A CS customer seemed to miss this warning before (see https://jira.mongodb.org/browse/CS-5372 ) so I revised the text to make it a little clearer. 

Author: matulef

source/release-notes/2.4.txt

@@ -365,7 +365,7 @@ key. Consider the following properties when using a hashed shard key:
   shard or set of shards; however, the :program:`mongos` must route 
   range queries to all shards.
 
-- When using a hashed shard key on a collection without data, MongoDB
+- When using a hashed shard key on a new collection, MongoDB
   automatically pre-splits the range of 64-bit hash values into
   chunks. By default, the initial number of chunks is equal to twice
   the number of shards at creation time. You can change the number of
@@ -375,6 +375,10 @@ key. Consider the following properties when using a hashed shard key:
   .. code-block:: javascript
 
      db.adminCommand( { shardCollection: "test.collection", key: { a: "hashed"}, numInitialChunks: 2001 } )
+  
+  Note: pre-splitting only occurs when sharding empty collections. The
+  chunks will not be pre-split when sharding collections that already
+  have data. 
 
 .. warning::