Documentation: dmaengine: clarify dma_slave_config expectations

dma_slave_config is expected to be set for slave operations Only, not for
memcpy ones

Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/dmaengine/provider.txt

@@ -310,6 +310,10 @@ supported.
      - Even though that structure contains a direction field, this
        field is deprecated in favor of the direction argument given to
        the prep_* functions
+     - This call is mandatory for slave operations only. This should NOT be
+       set or expected to be set for memcpy operations.
+       If a driver support both, it should use this call for slave
+       operations only and not for memcpy ones.
 
    * device_pause
      - Pauses a transfer on the channel