Document the detached_partitions table

db/docs/detached_partitions.yml

@@ -4,6 +4,14 @@ classes:
 - Postgresql::DetachedPartition
 feature_categories:
 - database
-description: TODO
+description: >
+  The detached_partitions table stores information about partitions in the gitlab_partitions_dynamic schema that
+  have been scheduled for drop, but not yet dropped.
+
+  These partitions were created by the Gitlab::Database::Partitioning::PartitionManager, and then detached from their
+  parent tables and added to this table when no longer part of the active partition set. Partitions are scheduled for
+  deletion one week after detachment.
+
+  Rows in this table are processed by Database::DropDetachedPartitionsWorker, which runs once a day.
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67056
 milestone: '14.2'