media: dmx-fread.rst: specify how DMX_CHECK_CRC works

In the past, the documentation used to say that, if a CRC error
was found, a "-ECRC" error would be returned. That's not true:
the DVB core will just silently ignore such errors.

So, add an explicit note about that.

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