esp_http_server: modify error print for clarifying internal socket usage

Closes https://github.com/espressif/esp-idf/issues/10108
Closes IDFGH-8663

components/esp_http_server/include/esp_http_server.h

@@ -155,7 +155,7 @@ typedef struct httpd_config {
      */
     uint16_t    ctrl_port;
 
-    uint16_t    max_open_sockets;   /*!< Max number of sockets/clients connected at any time*/
+    uint16_t    max_open_sockets;   /*!< Max number of sockets/clients connected at any time (3 sockets are reserved for internal working of the HTTP server) */
     uint16_t    max_uri_handlers;   /*!< Maximum allowed uri handlers */
     uint16_t    max_resp_headers;   /*!< Maximum allowed additional headers in HTTP response */
     uint16_t    backlog_conn;       /*!< Number of backlog connections */

components/esp_http_server/src/httpd_main.c

@@ -466,7 +466,7 @@ esp_err_t httpd_start(httpd_handle_t *handle, const httpd_config_t *config)
      * So the total number of required sockets is max_open_sockets + 3
      */
     if (CONFIG_LWIP_MAX_SOCKETS < config->max_open_sockets + 3) {
-        ESP_LOGE(TAG, "Configuration option max_open_sockets is too large (max allowed %d)\n\t"
+        ESP_LOGE(TAG, "Config option max_open_sockets is too large (max allowed %d, 3 sockets used by HTTP server internally)\n\t"
                  "Either decrease this or configure LWIP_MAX_SOCKETS to a larger value",
                  CONFIG_LWIP_MAX_SOCKETS - 3);
         return ESP_ERR_INVALID_ARG;