DOCS-120 edits to the examples in the replica set configuration setting

Author: Sam Kleinman

source/core/replication.txt

@@ -496,11 +496,13 @@ your replica set:
 - Consider keeping one or two members of the set in an off-site data
   center, but make sure to configure the :ref:`priority
   <replica-set-node-priority>` to prevent it from becoming primary.
-  
-- Create custom write concerns with 
-  :ref:`replica set tags <replica-set-configuration-tag-sets>` to ensure
-  data is written to specific members of the replica set or to facilitate
-  data center awareness.
+
+- Create custom write concerns with :ref:`replica set tags
+  <replica-set-configuration-tag-sets>` to ensure that applications
+  can control the threshold for a successful write operation. Use
+  these write concerns to ensure that operations propagate to specific
+  data centers or to machines of different functions before returning
+  successfully.
 
 For more information regarding replica set configuration and
 deployments see :doc:`/administration/replica-set-architectures`.