Document that an unset scheduler strategy whitelist no longer allows every strategy class

conf/defaults.yaml

@@ -92,6 +92,7 @@ nimbus.file.copy.expiration.secs: 600
 nimbus.topology.validator: "org.apache.storm.nimbus.DefaultTopologyValidator"
 # The scheduler strategies that a topology is allowed to select through topology.scheduler.strategy.
 # Defaults to the strategies shipped with Storm; a custom strategy must be added to this list.
+# Removing this config or leaving it unset selects the same shipped strategies, not all of them.
 nimbus.scheduler.strategy.class.whitelist:
   - "org.apache.storm.scheduler.resource.strategies.scheduling.DefaultResourceAwareStrategy"
   - "org.apache.storm.scheduler.resource.strategies.scheduling.DefaultResourceAwareStrategyOld"

docs/Resource_Aware_Scheduler_overview.md

@@ -251,7 +251,7 @@ A user can specify on a per topology basis what scheduling strategy to use.  Use
 Parameters:
 * clazz – The strategy class that implements the IStrategy interface
 
-Only the strategies that ship with Storm may be selected by default.  A strategy of your own has to be added to `nimbus.scheduler.strategy.class.whitelist` in the nimbus configuration, otherwise nimbus refuses to schedule the topology and reports which class was refused.
+Only the strategies that ship with Storm may be selected by default.  They are listed under `nimbus.scheduler.strategy.class.whitelist` in *conf/defaults.yaml*, and the same set applies when that config is not set at all.  This differs from earlier releases, where leaving `nimbus.scheduler.strategy.class.whitelist` unset allowed any strategy class on the nimbus classpath, so a cluster upgrading from such a release has to add its own strategies to the list.  A strategy of your own has to be added to `nimbus.scheduler.strategy.class.whitelist` in the nimbus configuration, otherwise nimbus refuses to schedule the topology and reports which class was refused.
 
 Example Usage:
 ```

storm-client/src/jvm/org/apache/storm/Config.java

@@ -2079,7 +2079,8 @@ public class Config extends HashMap<String, Object> {
     public static final String NIMBUS_IMPERSONATION_ACL = "nimbus.impersonation.acl";
     /**
      * A whitelist of the RAS scheduler strategies allowed by nimbus. Should be a list of fully-qualified class names. When it is not
-     * set only the scheduler strategies shipped with Storm are allowed, so a custom strategy has to be listed here explicitly.
+     * set only the scheduler strategies shipped with Storm are allowed, so a custom strategy has to be listed here explicitly. A
+     * topology that selects a strategy which is not allowed is rejected by nimbus.
      */
     @IsStringList
     public static final String NIMBUS_SCHEDULER_STRATEGY_CLASS_WHITELIST = "nimbus.scheduler.strategy.class.whitelist";