spi: spi.h: clarify the documentation of transfer_one

Explicitly note the transfer_one and transfer_one_message are mutually
exclusive, to make the text a little more newcomers friendly.
Signed-off-by: NBaruch Siach <baruch@tkos.co.il>
Signed-off-by: NMark Brown <broonie@linaro.org>

include/linux/spi/spi.h

@@ -287,7 +287,10 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  *                  - return 1 if the transfer is still in progress. When
  *                    the driver is finished with this transfer it must
  *                    call spi_finalize_current_transfer() so the subsystem
- *                    can issue the next transfer
+ *                    can issue the next transfer. Note: transfer_one and
+ *                    transfer_one_message are mutually exclusive; when both
+ *                    are set, the generic subsystem does not call your
+ *                    transfer_one callback.
  * @unprepare_message: undo any work done by prepare_message().
  * @cs_gpios: Array of GPIOs to use as chip select lines; one per CS
  *	number. Any individual value may be -ENOENT for CS lines that