media: Documentation: media: Describe V4L2_CID_UNIT_CELL_SIZE

New control to pass to userspace the width/height of a pixel. Which is
needed for calibration and lens selection.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>