V4L/DVB (9892): cx18: VBI comment corrections and comments about VBI issues

VBI comment corrections to avoid future confusion about standards.
Comments on cx18 VBI implementation shortcomings that need resolution.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>