Complete documentation on append-only remote repos (#5519)

improve docs/FAQ about append-only remote repos, fixes #5497

docs/faq.rst

@@ -210,6 +210,14 @@ This is not a problem anymore.
 
 For more details, see :ref:`checkpoints_parts`.
 
+How can I switch append-only mode on and off?
+-----------------------------------------------------------------------------------------------------------------------------------
+
+You could do that (via borg config REPO append_only 0/1), but using different
+ssh keys and different entries in ``authorized_keys`` is much easier and also
+maybe has less potential of things going wrong somehow.
+
+
 My machine goes to sleep causing `Broken pipe`
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