docs: say the realtime primary-read change is opt-in

Author: ericallam

.server-changes/realtime-run-reads-from-primary.md

@@ -3,4 +3,4 @@ area: webapp
 type: improvement
 ---
 
-Realtime run subscriptions can now read run data straight from the primary database, so a run's latest state is never served from a lagging replica.
+Realtime run subscriptions can now be configured to read run data straight from the primary database, so a run's latest state is never served from a lagging replica. Off by default; replica reads are unchanged unless you turn it on.