[media] DocBook: add a note about the ALSA API

The Docbook mistakenly makes to believe that all needed APIs for
media devices are there. Add a note there pointing that some
sub-devices are actually be controlled via ALSA API.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media_api.tmpl

@@ -72,6 +72,9 @@
 		<xref linkend="fe-delivery-system-t" />.</para>
 	<para>The third part covers the Remote Controller API.</para>
 	<para>The fourth part covers the Media Controller API.</para>
+	<para>It should also be noticed that a media device may also have audio
+	      components, like mixers, PCM capture, PCM playback, etc, with
+	      are controlled via ALSA API.</para>
 	<para>For additional information and for the latest development code,
 		see: <ulink url="http://linuxtv.org">http://linuxtv.org</ulink>.</para>
 	<para>For discussing improvements, reporting troubles, sending new drivers, etc, please mail to: <ulink url="http://vger.kernel.org/vger-lists.html#linux-media">Linux Media Mailing List (LMML).</ulink>.</para>