Update Container-Sync Docs for internal_client.conf

At patch [1], container sync daemon is updated to use internal client.
However, the "Configuring Container Sync" section [2] of swift docs doesn't
updated for this change.

This patch adds an explanation about the change.

[1]: https://review.openstack.org/#/c/452773/
[2]: http://docs.openstack.org/developer/swift/overview_container_sync.html#configuring-container-sync

Change-Id: Ia1aabeab250cbe27414d7877138ad38708f5ce3f
Closes-Bug: 1446721

doc/source/overview_container_sync.rst

@@ -98,6 +98,12 @@ container_sync section only needs the "use" item. For example::
     [filter:container_sync]
     use = egg:swift#container_sync
 
+The container sync daemon will use an internal client to sync objects. Even if
+you don't configure the internal client, the container sync daemon will work
+with default configuration. The default configuration is as same as
+``internal-client.conf-sample``. If you want to configure the internal client,
+please update ``internal_client_conf_path`` of container-server.conf. The
+configuration file at the path will be used for the internal client.
 
 -------------------------------------------------------
 Old-Style: Configuring a Cluster's Allowable Sync Hosts