Talk more about the threshold parameter of rebalance_table_shards()

## Communication goals (e.g. detailed howto vs orientation)

The parameter controls how precisely balanced shards should become. A value of 0.0 means perfectly balanced, but that takes more work by the rebalancer. The default is 0.1. It surprises some people when they see the imbalance, so we should update our examples to specify the threshold.

We should also mention the default value in our reference section if it doesn't already.

### Good locations for content in docs structure

* http://docs.citusdata.com/en/v8.1/admin_guide/cluster_management.html#rebalance-shards-without-downtime