add a comment block for us_ticker_set_interrupt()

libraries/mbed/targets/hal/TARGET_NORDIC/TARGET_MCU_NRF51822/us_ticker.c

@@ -168,6 +168,19 @@ uint32_t us_ticker_read()
     return RTC_UNITS_TO_MICROSECONDS(rtc1_getCounter());
 }
 
+/**
+ * Setup the us_ticker callback interrupt to go at the given timestamp.
+ *
+ * @Note: only one callback is pending at any time.
+ *
+ * @Note: If a callback is pending, and this function is called again, the new
+ * callback-time overrides any existing callback setting. It is the caller's
+ * responsibility to ensure that this function is called to setup a callback for
+ * the earliest timeout.
+ *
+ * @Note: If this function is used to setup an interrupt for 'right-now' or a
+ * time in the past, then the callback is invoked right-away.
+ */
 void us_ticker_set_interrupt(timestamp_t timestamp)
 {
     if (!us_ticker_inited) {