Update pg_dist_authinfo instructions to work with rebalancing

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

Our instructions should talk about setting this table on the workers too so they can send information between each other.

### Good locations for content in docs structure

http://docs.citusdata.com/en/v7.5/admin_guide/cluster_management.html#connection-management

## How does this work? (devs)

* Coordinator: add one for localhost (-1)
* Workers: (0 = all) but we want to more explicitly list all workers from each other so that it’s not wide open to anyone
* Other note, need to specify node_id but the worker names are not evident

### Example sql

### Corner cases, gotchas

We'll need to test a rebalance manually to ensure this works.

### Are there relevant blog posts or outside documentation about the concept/feature?

### Link to relevant commits and regression tests if applicable