[media] v4l2-framework.txt: document new v4l2_device release() callback

	state->instance = atomic_inc_return(&drv_instance) - 1;

}

If you have multiple device nodes then it can be difficult to know when it is

safe to unregister v4l2_device. For this purpose v4l2_device has refcounting

support. The refcount is increased whenever video_register_device is called and

it is decreased whenever that device node is released. When the refcount reaches

zero, then the v4l2_device release() callback is called. You can do your final

cleanup there.

If other device nodes (e.g. ALSA) are created, then you can increase and

decrease the refcount manually as well by calling:

void v4l2_device_get(struct v4l2_device *v4l2_dev);

or:

int v4l2_device_put(struct v4l2_device *v4l2_dev);

struct v4l2_subdev

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