delete/prune: add hint about undelete

src/borg/archiver/delete_cmd.py

@@ -64,8 +64,11 @@ class DeleteMixIn:
             """
         This command deletes archives from the repository.
 
-        Important: When deleting archives, repository disk space is **not** freed until
-        you run ``borg compact``.
+        Important:
+
+        - Repository disk space is **not** freed until you run ``borg compact``.
+        - You can use ``borg undelete`` to undelete archives, but only until
+          you run ``borg compact``.
 
         When in doubt, use ``--dry-run --list`` to see what would be deleted.
 

src/borg/archiver/prune_cmd.py

@@ -215,7 +215,11 @@ class PruneMixIn:
         The prune command prunes a repository by deleting all archives not matching
         any of the specified retention options.
 
-        Important: Repository disk space is **not** freed until you run ``borg compact``.
+        Important:
+
+        - Repository disk space is **not** freed until you run ``borg compact``.
+        - You can use ``borg undelete`` to undelete archives, but only until
+          you run ``borg compact``.
 
         This command is normally used by automated backup scripts wanting to keep a
         certain number of historic backups. This retention policy is commonly referred to as