media: admin-guide: improve cardlist.rst documentation

The cardlist section is important for some boards, because they
may require extra modprobe parameters.

Improve the docs to mention that.

Thanks-to: Hans Verkuil <hverkuil-cisco@xs4all.nl> # for providing me some PCI IDs
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>