intel_th: Document software sinks

Add documentation for the software sinks API of the MSU driver and the
msu-sink module in particular.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Link: https://lore.kernel.org/r/20191114064201.43089-3-alexander.shishkin@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>