mdns: Minor fix of API description and API usage

* Original commit: espressif/esp-idf@c297301

Author: david-cermak

components/mdns/include/mdns.h

@@ -546,6 +546,7 @@ esp_err_t mdns_query_async_delete(mdns_search_once_t* search);
  * @param search pointer to search object
  * @param timeout time in milliseconds to wait for answers
  * @param results pointer to the results of the query
+ * @param num_results pointer to the number of the actual result items (set to NULL to ignore this return value)
  *
  * @return
  *      True if search has finished before or at timeout

examples/protocols/mdns/main/mdns_example_main.c

@@ -146,7 +146,7 @@ static bool check_and_print_result(mdns_search_once_t *search)
 {
     // Check if any result is available
     mdns_result_t * result = NULL;
-    if (!mdns_query_async_get_results(search, 0, &result)) {
+    if (!mdns_query_async_get_results(search, 0, &result, NULL)) {
         return false;
     }