[doc] Add parameters for burn-in log files

Add the options to log the various burn-in steps to a file to the
documentation.

Change-Id: I10ca0a32b407a7f9d60827b4bde799a1688f5c1a

doc/source/admin/hardware-burn-in.rst

@@ -145,6 +145,30 @@ Then launch the test with:
 Both nodes will wait for the other node to show up and block while waiting.
 If the partner does not show up, the cleaning timeout will step in.
 
+Logging
+=======
+
+Since most of the burn-in steps are also providing information about the
+performance of the stressed components, keeping this information for
+verification or acceptance purposes may be desirable. By default, the
+output of the burn-in tools goes to the journal of the Ironic Python
+Agent and is therefore sent back as an archive to the conductor. In order
+to consume the output of the burn-in steps more easily, or even in real-time,
+the nodes can be configured to store the output of the individual steps to
+files in the ramdisk (from where they can be picked up by a logging pipeline).
+
+The configuration of the outpout file is done via one of
+``agent_burnin_cpu_outputfile``, ``agent_burnin_vm_outputfile``,
+``agent_burnin_fio_disk_outputfile``, and
+``agent_burnin_fio_network_outputfile`` parameters which need to be added
+to a node like:
+
+.. code-block:: console
+
+    baremetal node set --driver-info agent_burnin_cpu_outputfile=\
+        '/var/log/burnin.cpu' $NODE_NAME_OR_UUID
+
+
 Additional Information
 ======================