In Sharding Administration / Balancer Window, explain how to deactivate balancer window

In the documentation about sharding administration, we explain how to set a balancer window.

http://www.mongodb.org/display/DOCS/Sharding+Administration#ShardingAdministration-Balancerwindow

A customer asked how to turn off the balancer window option. This information is not in the docs. A sentence should be added that in order to turn it off, the "activeWindow" key needs to be $unset.