MINOR: add comment that timestamp unit is milliseconds (#13341)

Reviewers:  Divij Vaidya <diviv@amazon.com>

clients/src/main/java/org/apache/kafka/clients/consumer/ConsumerRecord.java

@@ -258,7 +258,7 @@ public class ConsumerRecord<K, V> {
     }
 
     /**
-     * The timestamp of this record
+     * The timestamp of this record, in milliseconds elapsed since unix epoch.
      */
     public long timestamp() {
         return timestamp;