V4L/DVB (5437): Update cx23415 documentation

Adds more osd mode switching information.
Corrects some information regarding mode selection & local alpha operation for
16 bit modes.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>