Documentation Error on `citus_rebalance_start`

[aytekinar]

Actual Behavior

The documentation has misleading information regarding the argument set of 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.

[ivyazmitinov]

I would also add that description of the block_writes shard transfer mode is incorrect: it states

Use COPY (blocking writes) for tables lacking primary key or replica identity.

While in reality it uses COPY and blocks writes for all tables in question, as can be seen here