stm32:f1:RTC: add documentation

include/libopencm3/stm32/f1/rtc.h

@@ -138,12 +138,18 @@
 
 /* TODO */
 
-/* --- Function prototypes --------------------------------------------------*/
-
+/** RTC Interrupt Flags */
 typedef enum {
-	RTC_SEC, RTC_ALR, RTC_OW,
+/** Counter Second Flag */
+	RTC_SEC,
+/** Alarm Event Flag */
+    RTC_ALR,
+/** Counter Overflow Flag */
+    RTC_OW,
 } rtcflag_t;
 
+/* --- Function prototypes --------------------------------------------------*/
+
 BEGIN_DECLS
 
 void rtc_awake_from_off(enum rcc_osc clock_source);