media: cec/core: clarify rx-arb-lost usage message

The rx-arb-lost error injection command only works with <op> set to 'any'.
Explicitly say so in the usage message.

Also use [] to indicate that the <poll> argument is optional.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>