DOCS-924: clarifiy connection pooling faq

Author: Sam Kleinman

source/faq/sharding.txt

@@ -194,11 +194,16 @@ How many connections does each ``mongos`` need?
 -----------------------------------------------
 
 Each client maintains a connection to a :program:`mongos` instance.
-Each :program:`mongos` instance maintains a pool of connections to
-the members of a replica set supporting the sharded cluster.
-Connections between :program:`mongos` and :program:`mongod` instances
-are used by one client at a time (requests are not multiplexed or
-pipelined) and then returned to the connection pool.
+Each :program:`mongos` instance maintains a pool of connections to the
+members of a replica set supporting the sharded cluster.  Clients use
+connections between :program:`mongos` and :program:`mongod` instances
+one at a time. Requests are not multiplexed or pipelined. When client
+requests complete, the :program:`mongos` returns the connection to the
+pool.
+
+See the :ref:`System Resource Utilization
+<system-resource-utilization>` section of the
+:doc:`/administration/ulimit` document.
 
 Why does ``mongos`` hold connections open?
 ------------------------------------------