master_disable_node documentation is not correct

Currently, master_disable_node docs describe it as:

```
The master_disable_node function is the opposite of master_activate_node. It marks a node as inactive in the Citus metadata table pg_dist_node, removing it from the cluster temporarily. The function also deletes all reference table placements from the disabled node. To reactivate the node, just run master_activate_node again.
```

The node is not "removed" from the cluster temporarily. INSERTs and SELECTs can still use the node. It removes reference table copies from the node and avoids creation of new shards on that node.

We should update the docs to represent this information more accurately.