spi: document the transfer_one spi_master callback

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/spi/spi-summary

@@ -545,6 +545,21 @@ SPI MASTER METHODS
 	spi_finalize_current_message() so the subsystem can issue the next
 	transfer. This may sleep.
 
+    master->transfer_one(struct spi_master *master, struct spi_device *spi,
+			 struct spi_transfer *transfer)
+	The subsystem calls the driver to transfer a single transfer while
+	queuing transfers that arrive in the meantime. When the driver is
+	finished with this transfer, it must call
+	spi_finalize_current_transfer() so the subsystem can issue the next
+	transfer. This may sleep. Note: transfer_one and transfer_one_message
+	are mutually exclusive; when both are set, the generic subsystem does
+	not call your transfer_one callback.
+
+	Return values:
+	negative errno: error
+	0: transfer is finished
+	1: transfer is still in progress
+
     DEPRECATED METHODS
 
     master->transfer(struct spi_device *spi, struct spi_message *message)