mbed_wait_api: add comments to warn the func will lock deep sleep

platform/mbed_wait_api.h

@@ -53,18 +53,32 @@ extern "C" {
  *  the accuracy of single precision floating point).
  *
  *  @param s number of seconds to wait
+ * 
+ *  @note
+ *    If the RTOS is present, this function always spins to get the exact number of microseconds, 
+ *    which potentially affects power (such as preventing deep sleep) and multithread performance. 
+ *    You can avoid it by using Thread::wait().
  */
 void wait(float s);
 
 /** Waits a number of milliseconds.
  *
  *  @param ms the whole number of milliseconds to wait
+ * 
+ *  @note
+ *    If the RTOS is present, this function always spins to get the exact number of microseconds, 
+ *    which potentially affects power (such as preventing deep sleep) and multithread performance. 
+ *    You can avoid it by using Thread::wait().
  */
 void wait_ms(int ms);
 
 /** Waits a number of microseconds.
  *
  *  @param us the whole number of microseconds to wait
+ * 
+ *  @note
+ *    If the RTOS is present, this function always spins to get the exact number of microseconds, 
+ *    which potentially affects power (such as preventing deep sleep) and multithread performance.
  */
 void wait_us(int us);