Documentation: dmaengine: fix the DMA_CTRL_ACK documentation

As discussed recently the meaning of DMA_CTRL_ACK is that a desc cannot be
reused by provider until the client acknowledges receipt, i.e. has has a
chance to establish any dependency chains. So update documentation

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>