gpio: uapi: document possible values of gpioevent_data.id

Clarify the possible values of event id, rather than requiring the
reader to infer.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

include/uapi/linux/gpio.h

@@ -490,7 +490,8 @@ struct gpioevent_request {
 /**
  * struct gpioevent_data - The actual event being pushed to userspace
  * @timestamp: best estimate of time of event occurrence, in nanoseconds
- * @id: event identifier
+ * @id: event identifier, one of %GPIOEVENT_EVENT_RISING_EDGE or
+ *  %GPIOEVENT_EVENT_FALLING_EDGE
  *
  * Note: This struct is part of ABI v1 and is deprecated.
  * Use &struct gpio_v2_line_event instead.