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>

Documentation/dmaengine/provider.txt

@@ -345,12 +345,12 @@ where to put them)
       that abstracts it away.
 
   * DMA_CTRL_ACK
-    - If set, the transfer can be reused after being completed.
-    - There is a guarantee the transfer won't be freed until it is acked
-      by async_tx_ack().
-    - As a consequence, if a device driver wants to skip the dma_map_sg() and
-      dma_unmap_sg() in between 2 transfers, because the DMA'd data wasn't used,
-      it can resubmit the transfer right after its completion.
+    - If clear, the descriptor cannot be reused by provider until the
+      client acknowledges receipt, i.e. has has a chance to establish any
+      dependency chains
+    - This can be acked by invoking async_tx_ack()
+    - If set, does not mean descriptor can be reused
+
 
 General Design Notes
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