media: dvb/intro: adjust the notices about optional hardware

Both CA and decoders are optional. Also, the presence or
absence has nothing to do on being a PCI card or not.

Nowadays, most hardware leaves the decoders to either the
GPU or to some ISP inside the SoC, instead of implementing
it inside the Digital TV part of the device.

So, change the wording to reflect the hardware changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>