Update the documentation on hacked backup client.

Fixes #5480.

docs/deployment/pull-backup.rst

@@ -1,5 +1,6 @@
 .. include:: ../global.rst.inc
 .. highlight:: none
+.. _pull_backup:
 
 =======================
 Backing up in pull mode

docs/faq.rst

@@ -616,7 +616,7 @@ C to delete all backups residing on S.
 These are your options to protect against that:
 
 - Do not allow to permanently delete data from the repo, see :ref:`append_only_mode`.
-- Use a pull-mode setup using ``ssh -R``, see :issue:`900`.
+- Use a pull-mode setup using ``ssh -R``, see :ref:`pull_backup` for more informations.
 - Mount C's filesystem on another machine and then create a backup of it.
 - Do not give C filesystem-level access to S.