Requesting update to existing process/documentation? Smarthost settings only for specific domains

There's some cPanel official documentation that hasn't been updated in 2 years. The latest WHM release seems to support "Smarthosts" in the WHM > Exim configuration page. What is unclear is how to only configure a selected number of domains (or cPanel accounts) to route via the specific "Smarthost" setting while the rest of the server and brand new accounts use the native SMTP setting. Can someone from cPanel staff update the documentation to solve for the above? (references " ome "Service Configuration "Exim Configuration Manager" but unclear how to mix the two)