lp ticker wrapper: Add undefined behavior warning

hal/LowPowerTickerWrapper.h

@@ -74,6 +74,9 @@ public:
      *
      * This stops to wrapper layer from using the microsecond ticker.
      * This should be called before using the low power ticker APIs directly.
+     *
+     * @warning: Make sure to suspend the LP ticker first (call ticker_suspend()),
+     * otherwise the behavior is undefined.
      */
     void suspend();
 

hal/mbed_lp_ticker_wrapper.h

@@ -53,6 +53,9 @@ const ticker_data_t *get_lp_ticker_wrapper_data(const ticker_data_t *data);
  *
  * Pass through all interrupts to the low power ticker and stop using
  * the microsecond ticker.
+ *
+ * @warning: Make sure to suspend the LP ticker first (call ticker_suspend()),
+ * otherwise the behavior is undefined.
  */
 void lp_ticker_wrapper_suspend(void);