periph/rtc: move struct tm note to the top

The remark is valid for the whole interface, not just `rtc_get_time`.

drivers/include/periph/rtc.h

@@ -11,6 +11,11 @@
  * @ingroup     driver_periph
  * @brief       Low-level RTC (Real Time Clock) peripheral driver
  *
+ * @note
+ * The values used for setting and getting the time/alarm should
+ * conform to the `struct tm` specification.
+ * Compare: http://pubs.opengroup.org/onlinepubs/7908799/xsh/time.h.html
+ *
  * @{
  * @file
  * @brief       Low-level RTC peripheral driver interface definitions
@@ -68,9 +73,6 @@ int rtc_get_time(struct tm *time);
  *
  * @note Any already set alarm will be overwritten.
  *
- * The values in `tm` are expected to conform to the `struct tm` specification.
- * Compare: http://pubs.opengroup.org/onlinepubs/7908799/xsh/time.h.html
- *
  * @param[in] time          The value to trigger an alarm when hit.
  * @param[in] cb            Callback executed when alarm is hit.
  * @param[in] arg           Argument passed to callback when alarm is hit.