lwip: Mark PPP officially supported

Closes https://github.com/espressif/esp-idf/issues/10249

components/lwip/Kconfig

@@ -703,13 +703,11 @@ menu "LWIP"
 
 
     menuconfig LWIP_PPP_SUPPORT
-        bool "Enable PPP support (new/experimental)"
+        bool "Enable PPP support"
         default n
         help
             Enable PPP stack. Now only PPP over serial is possible.
 
-            PPP over serial support is experimental and unsupported.
-
     config LWIP_PPP_ENABLE_IPV6
         bool "Enable IPV6 support for PPP connections (IPV6CP)"
         depends on LWIP_PPP_SUPPORT && LWIP_IPV6

docs/en/api-guides/lwip.rst

@@ -21,6 +21,7 @@ Some common lwIP "app" APIs are supported indirectly by ESP-IDF:
 - ICMP Ping is supported using a variation on the lwIP ping API. See :doc:`/api-reference/protocols/icmp_echo`.
 - NetBIOS lookup is available using the standard lwIP API. :example:`protocols/http_server/restful_server` has an option to demonstrate using NetBIOS to look up a host on the LAN.
 - mDNS uses a different implementation to the lwIP default mDNS (see :doc:`/api-reference/protocols/mdns`), but lwIP can look up mDNS hosts using standard APIs such as ``gethostbyname()`` and the convention ``hostname.local``, provided the :ref:`CONFIG_LWIP_DNS_SUPPORT_MDNS_QUERIES` setting is enabled.
+- The PPP implementation in lwIP can be used to create PPPoS (PPP over serial) interface in ESP-IDF. Please refer to the documentation of :doc:`/api-reference/network/esp_netif` component to create and configure a PPP network interface, by means of the ``ESP_NETIF_DEFAULT_PPP()`` macro defined in :component_file:`esp_netif/include/esp_netif_defaults.h`. Additional runtime settings are provided via the :component_file:`esp_netif/include/esp_netif_ppp.h`. PPPoS interfaces are typically used to interact with NBIoT/GSM/LTE modems; more application level friendly API is supported by `esp_modem <https://components.espressif.com/component/espressif/esp_modem>`_ library, which uses this PPP lwIP module behind the scenes.
 
 BSD Sockets API
 ---------------