V4L/DVB (9369): Documentation update for cx88

Attached is a patch that updates the cx88 documentation to add the fact the
closed caption works for at least NTSC capture.

ps: I also updated the wiki at:
http://www.linuxtv.org/v4lwiki/index.php/Text_capture#cx88_devices



Signed-off-by: Rafael Diniz <diniz@wimobilis.com.br>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>