spi: Document cs_gpios and cs_gpio in kernel-doc

This adds missing kernel-doc entries for cs_gpios in struct spi_master and
cs_gpio in struct spi_device.
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
[grant.likely: tweaked the language of the descriptions]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

include/linux/spi/spi.h

@@ -57,6 +57,8 @@ extern struct bus_type spi_bus_type;
  * @modalias: Name of the driver to use with this device, or an alias
  *	for that name.  This appears in the sysfs "modalias" attribute
  *	for driver coldplugging, and in uevents used for hotplugging
+ * @cs_gpio: gpio number of the chipselect line (optional, -EINVAL when
+ *	when not using a GPIO line)
  *
  * A @spi_device is used to interchange data between an SPI slave
  * (usually a discrete chip) and CPU memory.
@@ -258,6 +260,9 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
  * @unprepare_transfer_hardware: there are currently no more messages on the
  *	queue so the subsystem notifies the driver that it may relax the
  *	hardware by issuing this call
+ * @cs_gpios: Array of GPIOs to use as chip select lines; one per CS
+ *	number. Any individual value may be -EINVAL for CS lines that
+ *	are not GPIOs (driven by the SPI controller itself).
  *
  * Each SPI master controller can communicate with one or more @spi_device
  * children.  These make a small bus, sharing MOSI, MISO and SCK signals