Missing details in Rails migration section

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

* In a test suite the truncation database cleaner works best, and the rollback one has some issues
* `config.activerecord.schema_format` needs adjustment, and Samay will provide more info about this

### Good locations for content in docs structure

http://docs.citusdata.com/en/v7.5/develop/migration_mt_ror.html

## How does this work? (devs)

### Example sql

### Corner cases, gotchas

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

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