MINOR: Mention in configuration of broker setting log.retention.ms that -1 disables retention by time (#6464)

Includes an update to the relevant configuration doc.

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

@@ -597,7 +597,7 @@ object KafkaConfig {
   val LogRollTimeJitterMillisDoc = "The maximum jitter to subtract from logRollTimeMillis (in milliseconds). If not set, the value in " + LogRollTimeJitterHoursProp + " is used"
   val LogRollTimeJitterHoursDoc = "The maximum jitter to subtract from logRollTimeMillis (in hours), secondary to " + LogRollTimeJitterMillisProp + " property"
 
-  val LogRetentionTimeMillisDoc = "The number of milliseconds to keep a log file before deleting it (in milliseconds), If not set, the value in " + LogRetentionTimeMinutesProp + " is used"
+  val LogRetentionTimeMillisDoc = "The number of milliseconds to keep a log file before deleting it (in milliseconds), If not set, the value in " + LogRetentionTimeMinutesProp + " is used. If set to -1, no time limit is applied."
   val LogRetentionTimeMinsDoc = "The number of minutes to keep a log file before deleting it (in minutes), secondary to " + LogRetentionTimeMillisProp + " property. If not set, the value in " + LogRetentionTimeHoursProp + " is used"
   val LogRetentionTimeHoursDoc = "The number of hours to keep a log file before deleting it (in hours), tertiary to " + LogRetentionTimeMillisProp + " property"