Start section for tombstone retention

Author: kbatuigas

modules/manage/pages/cluster-maintenance/compaction-settings.adoc

@@ -26,6 +26,13 @@ Compaction policy may be applied to a cluster or to an individual topic. If both
 * `compact`: This triggers only cleanup of messages with multiple versions. A message that represents the only version for a given key is not deleted.
 * `compact,delete`: This combines both policies, deleting messages exceeding the retention period while compacting multiple versions of messages.
 
+== Tombstone marker removal
+
+When all messages for a given key are deleted, the topic partition keeps one message, called a tombstone, that contains the key, and a null value. A tombstone indicates to clients that the messages have been deleted. How long Redpanda keeps these tombstones depends on the topic cleanup policy:
+
+* For topics with a `compact,delete` policy, Redpanda deletes the tombstones when a topic reaches its retention limits.
+* For topics with a `compact` policy, Redpanda deletes the tombstones based on the specified tombstone retention policy. This retention policy is set with the `delete.retention.ms` topic configuration property. 
+
 == Compaction policy settings
 
 The various cleanup policy settings rely on proper tuning of a cluster's compaction and retention policy options. The applicable settings are: