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>