V4L/DVB (9482): Documentation, especially regarding audio and informational links

This patch adds a recommendation to select SND_USB_AUDIO for listing and
adds a documentation file for si470x.

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>