media: field-order.rst: clarify FIELD_ANY and FIELD_NONE

Rephrased the FIELD_ANY description: rather than explicitly list
field values, just say 'when any field format is acceptable'. The
list of FIELD values was outdated, so it was a bit confusing.

The FIELD_NONE description said that 'The driver may also indicate
this order when it cannot distinguish between V4L2_FIELD_TOP and
V4L2_FIELD_BOTTOM'. This is false, NONE really means a full frame
and userspace depends on that. So drop this line completely. There
are no drivers that do this anyway.

FIELD_TOP/BOTTOM/ALTERNATE indicate that the number of lines in the
buffer are that of a field, not frame, so returning NONE here would
cause huge problems.

Finally attempt to clarify 'progressive' and 'interlaced' a little
bit.

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