dmaengine: clarify the issue_pending expectations

Although Documentation explicitly mentions the expectations, the comment in
header can be lead to different expectation so clear up the confusion

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

include/linux/dmaengine.h

@@ -447,7 +447,8 @@ struct dmaengine_unmap_data {
  * 	communicate status
  * @phys: physical address of the descriptor
  * @chan: target channel for this operation
- * @tx_submit: set the prepared descriptor(s) to be executed by the engine
+ * @tx_submit: accept the descriptor, assign ordered cookie and mark the
+ * descriptor pending. To be pushed on .issue_pending() call
  * @callback: routine to call after this operation is complete
  * @callback_param: general parameter to pass to the callback routine
  * ---async_tx api specific fields---