doc: add documentation for RF calibration

Add an API to erase phy namespace of NVS

components/esp32/include/esp_phy_init.h

@@ -156,6 +156,18 @@ esp_err_t esp_phy_load_cal_data_from_nvs(esp_phy_calibration_data_t* out_cal_dat
  */
 esp_err_t esp_phy_store_cal_data_to_nvs(const esp_phy_calibration_data_t* cal_data);
 
+/**
+ * @brief Erase PHY calibration data which is stored in the NVS
+ *
+ * This is a function which can be used to trigger full calibration as a last-resort remedy 
+ * if partial calibration is used. It can be called in the application based on some conditions 
+ * (e.g. an option provided in some diagnostic mode).
+ *
+ * @return ESP_OK on success
+ * @return others on fail. Please refer to NVS API return value error number.
+ */
+esp_err_t esp_phy_erase_cal_data_in_nvs(void);
+
 /**
  * @brief Initialize PHY and RF module
  *