docs: how to debug borg mount, fixes #5461

src/borg/archiver/mount_cmds.py

@@ -122,6 +122,13 @@ class MountMixIn:
 
         When running in the foreground, ^C/SIGINT cleanly unmounts the filesystem,
         but other signals or crashes do not.
+
+        Debugging:
+
+        ``borg mount`` usually daemonizes and the daemon process sends stdout/stderr
+        to /dev/null. Thus, you need to either use ``-f / --foreground`` to make it stay
+        in the foreground and not daemonize, or use ``BORG_LOGGING_CONF`` to reconfigure
+        the logger to output to a file.
         """
         )
         subparser = subparsers.add_parser(