api: gpio: add missing member docstrings for gpio_dt_spec

This makes the breathe output for this type useful.

Signed-off-by: Martí Bolívar <[email protected]>

Author: mbolivar-nordic

include/drivers/gpio.h

@@ -325,15 +325,23 @@ typedef uint8_t gpio_dt_flags_t;
 typedef uint32_t gpio_flags_t;
 
 /**
- * @brief Provides a type to hold GPIO information specified in devicetree
+ * @brief Container for GPIO pin information specified in devicetree
  *
- * This type is sufficient to hold a GPIO device pointer, pin number,
- * and the subset of the flags used to control GPIO configuration
- * which may be given in devicetree.
+ * This type contains a pointer to a GPIO device, pin number for a pin
+ * controlled by that device, and the subset of pin configuration
+ * flags which may be given in devicetree.
+ *
+ * @see GPIO_DT_SPEC_GET_BY_IDX
+ * @see GPIO_DT_SPEC_GET_BY_IDX_OR
+ * @see GPIO_DT_SPEC_GET
+ * @see GPIO_DT_SPEC_GET_OR
  */
 struct gpio_dt_spec {
+	/** GPIO device controlling the pin */
 	const struct device *port;
+	/** The pin's number on the device */
 	gpio_pin_t pin;
+	/** The pin's configuration flags as specified in devicetree */
 	gpio_dt_flags_t dt_flags;
 };