The documentation is not so clear for newbies in a sense of what type of the
channels are supported by it.

Clarify this by adding a note at the preamble of the documentation.
Reported-by: N"Zhu, Tony" <tony.zhu@intel.com>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-By: NVinod Koul <vinod.koul@intel.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>