further warnings per @scotthernandez

Author: Ed Costello

source/reference/command/renameCollection.txt

@@ -52,17 +52,19 @@ renameCollection
       :dbcommand:`renameCollection` will fail if 
       `target` is the name of an existing collection
       and you do not specify ``dropTarget: true``.
-
-   .. warning:: 
 
       You cannot use :dbcommand:`renameCollection` with
       sharded collections.
-      
-   .. warning::
 
       If :dbcommand:`renameCollection` is interrupted prior to completion,
       the ``target`` collection and indexes will be left in an inconsistent
       state requiring manual intervention to clean up.
+      
+      A copy of a database across collections will impact performance for
+      the duration of the copy.
+      
+      You should not initiate :dbcommand:`renameCollection` while the 
+      source database is being written to. 
 
    The shell helper :method:`db.collection.renameCollection()` provides a
    simpler interface to using this command within a database.