Ticker class description update - small interval warning

Author: maciejbocianski

drivers/Ticker.h

@@ -100,6 +100,11 @@ class Ticker : public TimerEvent, private NonCopyable<Ticker> {
      *
      *  @param func pointer to the function to be called
      *  @param t the time between calls in micro-seconds
+     *
+     *  @note setting @a t to a value shorter that it takes to process the ticker callback
+     *  will cause the system to hang. Ticker callback will be called constantly with no time
+     *  for threads scheduling.
+     *
      */
     void attach_us(Callback<void()> func, us_timestamp_t t) {
         _function = func;