Update pg_dist_authinfo instructions to work with rebalancing

[begriffs]

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