forum: add section about deletion

service/forum.md

@@ -142,6 +142,15 @@ It's possible to trigger the creation of a downloadable backup via this [page](h
 on the admin interface. Once the backup is completed, a download link is sent by
 email to the admin who requested the backup.
 
+## Deleting topics and posts
+
+The design of Discourse is such that when a post is deleted via the web
+interface, it's merely flagged as such in the database. There is no purging of
+these records either, they remain for the life of the instance.
+
+To delete a post on Discourse requires direct modification of the database, which
+is obviously warned against by the developpers.
+
 ## Other documentation
 
  * https://meta.discourse.org/