While I am a general proponent of signposting text, I think it's useful to lead into a signpost with a a sentence about what the balancer does and why you'd want to configure the balancer at all, and then bury the signpost at the end of a context-setting paragraph. The elements of the context as I see it, are: 

1. The balancer runs on a single mongos instance
2. The balancer attempts to ensure that data, (i.e. chunks) are distributed evenly throughout the cluster.
3. In general the default balancer behavior is fine, but administrators may need to modify balancer behavior in some clusters depending on application or operational requirements. (i.e. backup, maintenance operations, ensuring quality of service during peak load, etc.) 

Not everything above is relevant, but a sentence or two that attempts to capture this may help...