Document clearing data from coordinator

[colton-citus]

If user takes the following steps, data will be left on coordinator:

1. Create table
2. Load data
3. Distribute table

Fix:

SET citus.enable_ddl_propagation TO false;
TRUNCATE TABLE target_table;
SET citus.enable_ddl_propagation TO true;

[jonels-msft]

The broader use case is when someone happened to load the data onto the coordinator prior to distributing it, and now wants to reclaim space on the coordinator?

I should check that we recommend distributing tables before loading data, and if not then I should probably update those instructions.

[colton-citus]

Yes, that is correct. I'd also suggest adding a warning and clearing steps so this doesn't surprise customers too often.