Disk threshold decider is enabled by default (elastic#50222)

An old comment had survived after the default was flipped.

Relates elastic#6204

server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java

@@ -19,8 +19,6 @@
 
 package org.elasticsearch.cluster.routing.allocation.decider;
 
-import java.util.Set;
-
 import com.carrotsearch.hppc.cursors.ObjectCursor;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
@@ -44,6 +42,8 @@
 import org.elasticsearch.index.Index;
 import org.elasticsearch.index.shard.ShardId;
 
+import java.util.Set;
+
 import static org.elasticsearch.cluster.routing.allocation.DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_HIGH_DISK_WATERMARK_SETTING;
 import static org.elasticsearch.cluster.routing.allocation.DiskThresholdSettings.CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING;
 
@@ -68,7 +68,7 @@
  * exact byte values for free space (like "500mb")
  *
  * <code>cluster.routing.allocation.disk.threshold_enabled</code> is used to
- * enable or disable this decider. It defaults to false (disabled).
+ * enable or disable this decider. It defaults to true (enabled).
  */
 public class DiskThresholdDecider extends AllocationDecider {