deep sleep: add note about disabling WiFi and BT

This change adds a note to esp_deep_sleep that applications should disable WiFi and BT before going into deep sleep.
SNTP example is also updated.

components/esp32/include/esp_deep_sleep.h

@@ -164,6 +164,14 @@ void esp_deep_sleep_start() __attribute__((noreturn));
  * Call to this function is equivalent to a call to esp_deep_sleep_enable_timer_wakeup
  * followed by a call to esp_deep_sleep_start.
  *
+ * esp_deep_sleep does not shut down WiFi, BT, and higher level protocol
+ * connections gracefully.
+ * Make sure relevant WiFi and BT stack functions are called to close any
+ * connections and deinitialize the peripherals. These include:
+ *     - esp_bluedroid_disable
+ *     - esp_bt_controller_disable
+ *     - esp_wifi_stop
+ *
  * This function does not return.
  *
  * @param time_in_us  deep-sleep time, unit: microsecond

examples/protocols/sntp/main/sntp_main.c

@@ -110,6 +110,8 @@ static void obtain_time(void)
         time(&now);
         localtime_r(&now, &timeinfo);
     }
+
+    ESP_ERROR_CHECK( esp_wifi_stop() );
 }
 
 static void initialize_sntp(void)