MINOR: Update docstring for "offsets.retention.minutes" config

Author: Manikumar Reddy O <manikumar.reddy@gmail.com>

Reviewers: Jason Gustafson <jason@confluent.io>

Closes #2562 from omkreddy/MINOR-DOC

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

@@ -587,7 +587,7 @@ object KafkaConfig {
   val OffsetsTopicPartitionsDoc = "The number of partitions for the offset commit topic (should not change after deployment)"
   val OffsetsTopicSegmentBytesDoc = "The offsets topic segment bytes should be kept relatively small in order to facilitate faster log compaction and cache loads"
   val OffsetsTopicCompressionCodecDoc = "Compression codec for the offsets topic - compression may be used to achieve \"atomic\" commits"
-  val OffsetsRetentionMinutesDoc = "Log retention window in minutes for offsets topic"
+  val OffsetsRetentionMinutesDoc = "Offsets older than this retention period will be discarded"
   val OffsetsRetentionCheckIntervalMsDoc = "Frequency at which to check for stale offsets"
   val OffsetCommitTimeoutMsDoc = "Offset commit will be delayed until all replicas for the offsets topic receive the commit " +
   "or this timeout is reached. This is similar to the producer request timeout."