Documentation Error on `citus_rebalance_start`

## Actual Behavior

The documentation has misleading information regarding the argument set of [`citus_rebalance_start`](https://docs.citusdata.com/en/v11.2/develop/api_udf.html#citus-rebalance-start), i.e., it lists the arguments `threshold`, `drain_only` and `rebalance_strategy` as opposed to the actual argument set `rebalance_strategy`, `drain_only` and `shard_transfer_mode`, in the given order.

## Desired Behavior

We should copy over the documentation regarding `shard_transfer_mode` from d1ff979, if the functionality has not changed, to the arguments of `citus_rebalance_start` and remove the non-existing `threshold` from the docs.

I can create a PR if desired.

/cc @begriffs, @jonels-msft.