media: v4l: doc: Clarify v4l2_mbus_fmt height definition

The v4l2_mbus_fmt width and height corresponds directly with the
v4l2_pix_format definitions, yet the differences in documentation make
it ambiguous what to do in the event of field heights.

Clarify this using the same text as is provided for the v4l2_pix_format
which is explicit on the matter, and by matching the terminology of
'image height' rather than the misleading 'frame height'.

Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>