Update KafkaConfig.scala (apache#7113)

Better clarify the auto leader rebalance config documentation to reflect additional leader.imbalance.per.broker.percentage config description.

Reviewers: Stanislav Kozlovski <[email protected]>, Jun Rao <[email protected]>

core/src/main/scala/kafka/server/KafkaConfig.scala

@@ -688,7 +688,7 @@ object KafkaConfig {
   val FetchPurgatoryPurgeIntervalRequestsDoc = "The purge interval (in number of requests) of the fetch request purgatory"
   val ProducerPurgatoryPurgeIntervalRequestsDoc = "The purge interval (in number of requests) of the producer request purgatory"
   val DeleteRecordsPurgatoryPurgeIntervalRequestsDoc = "The purge interval (in number of requests) of the delete records request purgatory"
-  val AutoLeaderRebalanceEnableDoc = "Enables auto leader balancing. A background thread checks and triggers leader balance if required at regular intervals"
+  val AutoLeaderRebalanceEnableDoc = "Enables auto leader balancing. A background thread checks the distribution of partition leaders at regular intervals, configurable by `leader.imbalance.check.interval.seconds`. If the leader imbalance exceeds `leader.imbalance.per.broker.percentage`, leader rebalance to the preferred leader for partitions is triggered."
   val LeaderImbalancePerBrokerPercentageDoc = "The ratio of leader imbalance allowed per broker. The controller would trigger a leader balance if it goes above this value per broker. The value is specified in percentage."
   val LeaderImbalanceCheckIntervalSecondsDoc = "The frequency with which the partition rebalance check is triggered by the controller"
   val UncleanLeaderElectionEnableDoc = "Indicates whether to enable replicas not in the ISR set to be elected as leader as a last resort, even though doing so may result in data loss"