## Summary
- Clarifies in the MySQL docs intro that read replica configuration is
fully independent — no values are inherited from the primary config
- Adds explicit callout that `mysql_read_replica_region` must be set
separately when using IAM authentication
- Adds note to `mysql_tls_config` explaining that this setting is
typically not needed for RDS IAM auth since Fleet uses bundled RDS CA
certificates (including GovCloud regions)
These changes address real-world customer confusion where a GovCloud
deployment failed because `FLEET_MYSQL_READ_REPLICA_REGION` was not set
independently of `FLEET_MYSQL_REGION`.
Note: related to #39832
## Testing
Documentation-only change — no code impact.
