media: Documentation: media: Document V4L2_CTRL_TYPE_AREA

A struct v4l2_area containing the width and the height of a rectangular
area.

Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
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>