media: vidioc-g-ext-ctrls.rst: document V4L2_CTRL_CLASS_DETECT

V4L2_CTRL_CLASS_DETECT was never documented here, add it.

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