esp_hw_support/clk_cali: fix xtal32k error detect

components/soc/esp32/include/soc/rtc.h

@@ -394,6 +394,11 @@ uint32_t rtc_clk_apb_freq_get(void);
  * 32k XTAL is being calibrated, but the oscillator has not started up (due to
  * incorrect loading capacitance, board design issue, or lack of 32 XTAL on board).
  *
+ * @note When 32k CLK is being calibrated, this function will check the accuracy
+ * of the clock. Since the xtal 32k or ext osc 32k is generally very stable, if
+ * the check fails, then consider this an invalid 32k clock and return 0. This
+ * check can filter some jamming signal.
+ *
  * @param cal_clk  clock to be measured
  * @param slow_clk_cycles  number of slow clock cycles to average
  * @return average slow clock period in microseconds, Q13.19 fixed point format,