Multi DB docs improvements
Multi DB docs improvements

- Clarify what the `replica` option does.
- Introduce `prevent_writes`, and remind the user that it's not perfect and the database should be configured as a replica too.
BlakeWilliams/rails,yawboakye/rails,yahonda/rails,Erol/rails,joonyou/rails,joonyou/rails,yahonda/rails,esparta/rails,joonyou/rails,yawboakye/rails,Envek/rails,rails/rails,esparta/rails,rails/rails,rails/rails,lcreid/rails,Stellenticket/rails,Erol/rails,yawboakye/rails,Envek/rails,kmcphillips/rails,joonyou/rails,EmmaB/rails-1,Vasfed/rails,Erol/rails,Vasfed/rails,Vasfed/rails,kddeisz/rails,MSP-Greg/rails,BlakeWilliams/rails,Edouard-chin/rails,rails/rails,BlakeWilliams/rails,georgeclaghorn/rails,Edouard-chin/rails,Stellenticket/rails,georgeclaghorn/rails,yawboakye/rails,MSP-Greg/rails,Vasfed/rails,Erol/rails,Envek/rails,lcreid/rails,kmcphillips/rails,Stellenticket/rails,kmcphillips/rails,MSP-Greg/rails,Edouard-chin/rails,Edouard-chin/rails,yahonda/rails,kddeisz/rails,esparta/rails,MSP-Greg/rails,esparta/rails,georgeclaghorn/rails,kmcphillips/rails,kddeisz/rails,Envek/rails,EmmaB/rails-1,EmmaB/rails-1,yahonda/rails,Stellenticket/rails,BlakeWilliams/rails,georgeclaghorn/rails,lcreid/rails,kddeisz/rails,lcreid/rails