media: cec-func-poll.rst/func-poll.rst: update EINVAL description

nfds depends on RLIMIT_NOFILE, not OPEN_MAX. Update the description
for cec and v4l2.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>