[media] DVB: dvb-net, make the kconfig text helpful

Telling the user they can disable an option if they want is not the
much useful. Describe what it is good for instead.

The text was derived from Mauro's email.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: Hans Petter Selasky <hselasky@c2i.net>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Hans Petter Selasky <hselasky@c2i.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>