[media] videobuf2-core.h: improve documentation

 *			queue setup from completing successfully; optional.

 * @buf_prepare:	called every time the buffer is queued from userspace

 *			and from the VIDIOC_PREPARE_BUF ioctl; drivers may

 *			perform any initialization required before each hardware

 *			operation in this callback; drivers that support

 *			VIDIOC_CREATE_BUFS must also validate the buffer size;

 *			if an error is returned, the buffer will not be queued

 *			in driver; optional.

 *			perform any initialization required before each

 *			hardware operation in this callback; drivers can

 *			access/modify the buffer here as it is still synced for

 *			the CPU; drivers that support VIDIOC_CREATE_BUFS must

 *			also validate the buffer size; if an error is returned,

 *			the buffer will not be queued in driver; optional.

 * @buf_finish:		called before every dequeue of the buffer back to

 *			userspace; drivers may perform any operations required

 *			before userspace accesses the buffer; optional. The

 *			buffer state can be one of the following: DONE and

 *			ERROR occur while streaming is in progress, and the

 *			PREPARED state occurs when the queue has been canceled

 *			and all pending buffers are being returned to their

 *			default DEQUEUED state. Typically you only have to do

 *			something if the state is VB2_BUF_STATE_DONE, since in

 *			all other cases the buffer contents will be ignored

 *			anyway.

 *			userspace; the buffer is synced for the CPU, so drivers

 *			can access/modify the buffer contents; drivers may

 *			perform any operations required before userspace

 *			accesses the buffer; optional. The buffer state can be

 *			one of the following: DONE and ERROR occur while

 *			streaming is in progress, and the PREPARED state occurs

 *			when the queue has been canceled and all pending

 *			buffers are being returned to their default DEQUEUED

 *			state. Typically you only have to do something if the

 *			state is VB2_BUF_STATE_DONE, since in all other cases

 *			the buffer contents will be ignored anyway.

 * @buf_cleanup:	called once before the buffer is freed; drivers may

 *			perform any additional cleanup; optional.

 * @start_streaming:	called once to enter 'streaming' state; the driver may