docs(wifi_provisioning): Use heap to allocate memory for the response

Added a note to specify that the memory for the response of a custom endpoint
should be allocated using the heap, as this memory gets freed by the protocomm layer
once it has been passed to the transport layer.

Closes https://github.com/espressif/esp-idf/issues/13263

examples/provisioning/wifi_prov_mgr/main/app_main.c

@@ -230,6 +230,8 @@ static void get_device_service_name(char *service_name, size_t max)
 /* Handler for the optional provisioning endpoint registered by the application.
  * The data format can be chosen by applications. Here, we are using plain ascii text.
  * Applications can choose to use other formats like protobuf, JSON, XML, etc.
+ * Note that memory for the response buffer must be allocated using heap as this buffer
+ * gets freed by the protocomm layer once it has been sent by the transport layer.
  */
 esp_err_t custom_prov_data_handler(uint32_t session_id, const uint8_t *inbuf, ssize_t inlen,
                                           uint8_t **outbuf, ssize_t *outlen, void *priv_data)