compact: explain more

src/borg/archiver/compact_cmd.py

@@ -185,9 +185,23 @@ def build_parser_compact(self, subparsers, common_parser, mid_common_parser):
             - interrupted backups (maybe retry the backup first before running compact!)
             - backup of source files that had an I/O error in the middle of their contents
               and that were skipped due to this.
+            - corruption of the repository (e.g. the archives directory having lost entries)
 
-            Important: after compacting it is not possible anymore to use ``borg undelete``
-            to recover previously deleted archives.
+            You usually don't want to run ``borg compact`` after every write operation, but
+            either regularly (e.g. once a month, possibly together with ``borg check``) or
+            when disk space needs to be freed.
+
+            **Important:**
+
+            After compacting it is not possible anymore to use ``borg undelete`` to recover
+            previously deleted archives.
+
+            ``borg compact`` might also delete data from archives that were "lost" due to
+            archives directory corruption. Such archives could potentially be restored with
+            ``borg check --find-lost-archives [--repair]``, which is slow and thus you
+            maybe usually don't want to do that unless there are signs of lost archives
+            (e.g. when seeing fatal errors when creating backups or when archives are
+            missing in ``borg list``).
 
             Differently than borg 1.x, borg2's compact needs the borg key if the repo is
             encrypted.