Correct the description of RS monitoring for read prefs section: not everyone uses isMaster for this purpose

Author: ajdavis

source/applications/replication.txt

@@ -496,9 +496,9 @@ Member Selection
 ````````````````
 
 Clients, by way of their drivers, and :program:`mongos` instances for
-sharded clusters periodically call :dbcommand:`isMaster` on all members of the
-replica set to update their view of the set's state, and to determine latency
-from the application to each :program:`mongod` instance.
+sharded clusters periodically update their view of the set's state: which
+members are up or down, which is primary, and the latency to each
+:program:`mongod` instance.
 
 For any operation that targets a member *other* than the
 :term:`primary`, the driver: