intel_th: Update ABI documentation

Commit a753bfcf ("intel_th: Make the switch allocate its subdevices")
changed the behavior so that the output port devices are created only for
the ports reported by the hardware and their initial state is "unassigned"
until a corresponding output port driver is loaded. Reflect this fact in
the ABI documentation.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reported-by: Ricardo Neri <ricardo.neri@intel.com>

Documentation/ABI/testing/sysfs-bus-intel_th-output-devices

@@ -3,11 +3,13 @@ Date:		June 2015
 KernelVersion:	4.3
 Contact:	Alexander Shishkin <alexander.shishkin@linux.intel.com>
 Description:	(RW) Writes of 1 or 0 enable or disable trace output to this
-		output device. Reads return current status.
+		output device. Reads return current status. Requires that the
+		correstponding output port driver be loaded.
 
 What:		/sys/bus/intel_th/devices/<intel_th_id>-msc<msc-id>/port
 Date:		June 2015
 KernelVersion:	4.3
 Contact:	Alexander Shishkin <alexander.shishkin@linux.intel.com>
 Description:	(RO) Port number, corresponding to this output device on the
-		switch (GTH).
+		switch (GTH) or "unassigned" if the corresponding output
+		port driver is not loaded.