backlight: backlight: Supply description for function args in existing Kerneldocs

Kerneldoc syntax is used, but not complete.  Descriptions required.

Prevents warnings like:

 drivers/video/backlight/backlight.c:329: warning: Function parameter or member 'reason' not described in 'backlight_force_update'
 drivers/video/backlight/backlight.c:354: warning: Function parameter or member 'props' not described in 'backlight_device_register'

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Jamey Hicks <jamey.hicks@hp.com>
Cc: Andrew Zabolotny <zap@homelink.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/backlight.c

@@ -320,6 +320,7 @@ ATTRIBUTE_GROUPS(bl_device);
  * backlight_force_update - tell the backlight subsystem that hardware state
  *   has changed
  * @bd: the backlight device to update
+ * @reason: reason for update
  *
  * Updates the internal state of the backlight in response to a hardware event,
  * and generate a uevent to notify userspace
@@ -344,6 +345,7 @@ EXPORT_SYMBOL(backlight_force_update);
  * @devdata: an optional pointer to be stored for private driver use. The
  *   methods may retrieve it by using bl_get_data(bd).
  * @ops: the backlight operations structure.
+ * @props: pointer to backlight's properties structure.
  *
  * Creates and registers new backlight device. Returns either an
  * ERR_PTR() or a pointer to the newly allocated device.