[media] DocBook media: Clarify V4L2_FIELD_ANY for drivers

Documentation for enum v4l2_field did not make it clear that V4L2_FIELD_ANY
is only acceptable as input to the kernel, not as a response from the
driver.

Make it clear, to stop userspace developers like me assuming it can be
returned by the driver.

Signed-off-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>