Document new citus.replication_model GUC.

## Why are we implementing it? (sales eng)

### What are the typical use cases?

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

### Good locations for content in docs structure
Configuration Reference.
We should also reference it in Cluster Management, where we talk about node failures.

## How does this work? (devs)

### Example sql

### Corner cases, gotchas

### Are there relevant blog posts or outside documentation about the concept/feature?
https://www.citusdata.com/blog/2016/12/15/citus-replication-model-today-and-tomorrow/ talks about why we made the change.

### Link to relevant commits and regression tests if applicable
https://github.com/citusdata/citus/commit/56197dbdba57a67ad170b97acf5f11254b85f008