docs: add another case of attempted hardlink usage

docs/usage/general/file-systems.rst.inc

@@ -23,4 +23,6 @@ and readable after one of the failures mentioned above occurred, run
   Large repositories may require large files (>2 GB).
 - Up to 1000 files per directory (10000 for repositories initialized with Borg 1.0)
 - mkdir(2) should be atomic, since it is used for locking
-- Hardlinks are needed for :ref:`borg_upgrade` (if ``--inplace`` option is not used)
+- Hardlinks are needed for :ref:`borg_upgrade` (if ``--inplace`` option is not used).
+  Also hardlinks are used for more safe and secure file updating (e.g. of the repo
+  config file), but the code tries to work also if hardlinks are not supported.